Open Question: Difficult basic physics question?

A 2.0kg block rides on top of a 6.0kg block as they oscillate along a frictionless surface. The spring constant is 25 N/m and the coefficient of static friction between the blocks is 0.6. What is the maximum oscillation amplitude for which the upper box doesn't slip?

|~~~~~M (M = m1 + m2; ~ indicates spring; | is the wall)

I have no idea how to solve this problem but I believe this involves looking at free body diagrams then calculating the Fs min = 11.76N; therefore the Fmax of the spring should be 11.76 N or greater?

F(x) = -kx = ma?

Full solution would be appreciated.

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