Assuming you are referring to the area of a “trapezoid”; in which one calculates the Area, “A”, as follows:
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 A = 1/2* h(b1+b2) ;

in which: A = Area = 16 (given); 
               h = height = 4 (given);
               b1 = length of one of the two bases = 3 (given);
               b2 = length of the other of the two bases = ? (what we want to solve                                                                                            for) ;
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Using the formula: 

A = 1/2 h(b1+b2) ;
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Let us plug in our known values:
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 →  16 = (1/2) * 4*(3 + b2) ;  → Solve for “b2”.
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 →Note: On the “right-hand side” on this equation: “(1/2)*(4) = 2 .” 
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 So, we can rewrite the equation as:
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 → 16 =   2*(3 + b2) ;  → Solve for “b2”.
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We can divide EACH side of the equation by “2”; to cancel the “2” on the “right-hand side” of the equation:
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 → 16 / 2 =   [2*(3 + b2)] / 2  ;  → to get:
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8 = (3 + b2) ;
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 → Rewrite as: 8 = 3 + b2;
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Subtract “3” from EACH side of the equation; to isolate “b2” on one side of the equation; and to solve for “b2” :
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 → 8 – 3 = 3 + b2 – 3 ;  → to get:
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b2 = 5;  From the 2 (TWO) answer choices given, this value,
“b2 = 5”, corresponds with the following answer choice:
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b2= [16-6]/2= 5 ; as this is the only answer choice that has: “b2 = 5”.
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As far getting “

b2 = 5″  from: “b2= [16-6]/2= 5”; (as mentioned in the answer choice), we need simply to approach the problem in a slightly different manner.  Let us do so, as follows:
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Start from: 
A = 1/2 h(b1+b2); and substitute our known (given) values):
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→ 16 = (1/2) *4 (3 + b2) ; → Solve for “b2”.
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Note that: (½)*4 = 2;  so we can substitute “2” for: “(1/2) *4” ; 
and rewrite the equation as follows:
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→ 16 = 2 (3 + b2) ;
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Note: The distributive property of multiplication:
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a*(b+c) = ab + ac ;
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As such: 2*(3 + b2) = (2*3 + 2*b2) = (6 + 2b2). 
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So we can substitute: “(6 + 2b2)” in lieu of “[2*(3 + b2)]”; and can rewrite the equation:
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→ 16 = 6 + 2(b2) ; Now, we can subtract “6” from EACH side of the equation; to attempt to isolate “b2” on one side of the equation:
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→ 16 – 6 =  6 + 2(b2) – 6 ;
      → Since “6-6 = 0”; the “6 – 6” on the “right-hand side” of the equation cancel.
→ We now have: 16 – 6 = 2*b2 ; 
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Now divide EACH SIDE of the equation by “2”; to isolate “b2” on one side of the equation; and to solve for “b2”:
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   → (16 – 6) / 2 = (2*b2) / 2 ; 
     → (16 – 6) / 2 = b2 ;
       → (10) / 2 = b2 = 5.
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NOTE: The other answer choice given: 
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16= 1/2* 4(3+b2)= 6+2b2″ is incorrect; since it does not solve for “b2”.