Answer:
  a = 4.0 m / s²,   so the result of the exercise is correct
Explanation:
This is an exercise on Newton's Second Law
        F = m a
we create a reference system with the horizontal x axis and the vertical y axis
Axis y
       N -W = 0
X axis
       F -fr = m a
give us the applied force F = 40 N, the value of the friction outside fr = 24 N
        a = (F - fr) / m
    Â
let's calculate
       a = (40 - 24) / 4
       a = 4.0 m / s²
so the result of the exercise is correct