Spring question



I'm having lots of trouble with this one, help would be much appreciated!

A spring of stiffness k supports a box of mass M, in which is placed a block of mass m. If the system is pulled downward a distance d from the equilibrium position and then released, find the force of reaction between the block and the bottom of the box as a function of time. For what value of d will the block just begin to leave the bottom of the box at the top of the verticle oscillations? Neglect any air resistance.

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