The net force acting on the mass parallel to the surface is
∑ F[horizontal] = F[applied] - F[friction] = ma
where m = 10 kg. Given that F[friction] = 6 N and a = 1.2 m/s², we have
F[applied] - 6 N = (10 kg) (1.2 m/s²)
F[applied] = 12 N + 6 N
F[applied] = 18 N
