The net force on the box perpendicular to the floor is
∑ F[perp] = F[normal] - mg = 0
where mg is the weight of the box. Then
F[normal] = (35.5 kg) g = 347.9 N
Since the box slides with constant speed, the net force parallel to the floor is
∑ F[para] = F[applied] - F[friction] = 0
We have
F[friction] = 0.15 F[normal] = 52.185 N
so that
F[applied] = F[friction] ≈ 52 N