Simple Harmonic system with spring, 10 pts.?

A block slides along a frictionless horizontal surface at speed v = 0.40 m/s. It runs into a spring and compresses it by 10 cm before coming momentarily to rest (and then going back the other direction). What time elapses between the block first hitting the spring and when it comes momentarily to rest?

I just used - .4 x 10 = -4. Hooke's Law(f = -kx), that wasn't right though.

Anyone know how to do this? :<