Weight calculation:

The weight by definition, is given by:

W = m * g

Where,

m: mass of the book box

g: acceleration of gravity

Substituting values we have:

W = 12 * 9.8

W = 117.6 N

Calculation of normal force:

For normal force, we make a free-body diagram in vertical direction, therefore, we have to:

Fn – W = 0

Rewriting we have:

Fn = W

Substituting values:

Fn = 117.6 N

Calculation of the friction force:

For the force of friction, we make a diagram of free body in horizontal direction, therefore, we have to:

96 – Fs = 0

Note: we equate to zero because the speed is constant, therefore, the acceleration is equal to zero.

Clearing we have:

Fs = 96

Calculation of the coefficient of friction:

By definition, we have to:

Fs = μFn

Clearing the coefficient of friction we have:

μ = Fs / Fn

Substituting values:

μ = 96 / 117.6

μ = 0.82