Physic

A block of mass 2.5 kg is placed on a frictionless inclined ramp of mass 17.8 kg, with the angle θ= 15o
Calculate the size of the net horizontal force that must be applied to the ramp to prevent the block from accelerating with respect to the ramp?

I don't understand this question at all, my answer is like 23 N, but it's wrong... I gues I have to find the force of ramp as well...... BTW, there's a diagram showing that the ramp has a force that's pushing it, but didn't say how much