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.