Ahorizontal force of 70 n is required to push a bookcase across a floor at a constant velocity. what is the net force acting on the bookcase?

The net force acting on the bookcase is zero.

The key element in the text of the problem is "at constant velocity". In fact, constant velocity means zero acceleration. But Newton's second law of motion states that the product between the acceleration of an object and its mass is equal to the net force acting on it:

Therefore, zero acceleration means a net force equal to zero.

weight of the box is given as

here this is placed on the inclined of angle 42 degree

now this weight will have two components one is along the incline other is perpendicular to it

so here we need to apply a force parallel to incline opposite to its component of weight

so here the component of weight is given as

here we have

so it requires 75 n force to move it upwards