This is Problem 10.46 from the Engineering Physics Book by Knight.
A vertical spring with k = 490 N/m is standing on the ground. You are holding a 5.0 kg block just above the spring, not quite touching it.
a.) How far does the spring compress if you let go of the block suddenly?
b.)...
You're right on your equation, however when you put the numbers in you're dividing m+M by M, when you should be using m the mass of the ball. so it's
(10+.006)/.006,
not
(10+.006)/10