Friction on two blocks simple help

[vsage]

OK I was helping a friend with his physics HW but I can't remember for the life of me how to do this.

A block of mass 4.0 kg is put on top of a block of mass M = 5.0 kg. To cause the top block to slip on the bottom one, while the bottom one is held fixed, a horizontal force of at least 17 N must be applied to the top block. The assembly of blocks is now placed on a horizontal, frictionless table

Find the magnitude of the maximum horizontal force F that can be applied to the lower block so that the blocks will move together.

Any hints?

[e(ho0n3]

Draw a free body diagram of the situation. You need to remember that static friction is the one keeping the top block and the bottom block together, so keep this in mind when setting up the equations.

[vsage]

Thanks. Nevermind we had a right answer previously but the computer wasn't accepting 38.24 as an answer but listed the answer as 38.2. Stupid huh?