The mass slows to a rest from an initial speed of 10.0 m/s in a matter of 1.50 s. If we assume constant acceleration, then the mass has acceleration a such that
0 = 10.0 m/s + a (1.50 s) ⇒ a ≈ -(10.0 m/s)/(1.50 s)
The net force acting on the mass as it slows down is
∑ F[horizontal] = -F[friction] = ma
where m = 15.0 kg, and we take the direction in which friction is acting to be negative. Then
-F[friction] = -(15.0 kg) (10.0 m/s)/(1.50 s) ⇒ F[friction] = 100 N
