Physics help?

At a waterpark, sleds with riders are sent along a slippery, horizontal surface by the release of a large, compressed spring. The spring with a force constant k = 4200 N/m and negligible m rests on the frictionless horizontal surface. One end is in contact with a stationary wall. A sled and rider with total m 73.0 kg are pushed against the other end, compressing the spring 0.400 m. The sled is then released with zero initial velocity. a) What is the sled's speed when the spring returns to its uncompressed length? b) What is the sled's speed when the spring is still compressed 0.220 m?