If manuel weighs 375 n and is holding the box. how much force is the floor exerting on him? (include direction)

The box is exerting a force of its weight. So we'll use W=mg (Note: you can use the notation F=ma, doesn't matter since weight IS a force.) We can call g negative since it's directed down towards the earth.

W = 5kg (-9.8 m / (s^2)) = - 49 N

Thus, the box exerts a force of 49N downwards.