The correct answer is A.
The given systems of equations are
If we multiply the first equation by 7 we get,
If we multiply the
Adding equation (3) and (4) gives,
We can see that f has been eliminated from the equation, and hence we can find g and use it to find f.
However, if we multiply the first equation by 3 and the second equation by 5, and then subtract, none of the variables will be eliminated.
Also, if we multiply the first equation by –7 and the second equation by 2, and then add, neither f nor g will be eliminated.
The same thing applies to multiplying the first equation by –3 and the second equation by 5, and then adding will not eliminate any of the variables.