AP Physics C question.. help?

Question

Every surface is frictionless in this problem.
A block of mass M is rolling on a frictionless plane (it is on wheels that have no mass). A horizontal force (F) is acting on it. On the top of block of mass M is a smaller block with mass M1. There another block with mass M2 (same as M1) is hanging off the front of block with mass M. It is being held up with a string that goes over a pully and is attached to block with mass M1.
What horizontal force, F, would be needed be applied to block of mass M to have the system of block M1 and M2 have an acceleration of 0?

Answer 1

If I'm reading this right then whatever force you had pushing it at the start will be the same force required to stop it but in the opposite direction, so -F Newtons.

Or if your just pushing it to begin with, then for an acceleartion of 0, F = ma, so the force would be 0.

I'm not sure what you mean by the question.