Help with Newton's Laws (AP Physics B)

[stonecoldgen]

Homework Statement

they keep telling me it is a simple problem, I've actually done harder problems of the same thing, but i just don't seem to get this one.

A 15kg block sits on a 25kg block. The coefficient of static friction between the blocks is 0.408 and there is no friction between the 25kg block and the floor. Find the maximum net force that can be applied to the 25kg blcok before the 15kg blcok begins to slide.

Homework Equations

sigmaF=a(sigmaM)
fr=(mu)g

The Attempt at a Solution

I think i need to find out at which point the exerted force surpases the friction force (because it is what holds the 2 blocks together).

This didn't work exactly how i wanted and it didn't give me the right answer.

 

[vela]

What force causes the top block to accelerate? What's the maximum acceleration the top block can have?