I have a momentum question but i just can't seem to figure it out.?

The launching mechanism of a toy gun consists of a spring of unknown spring constant. By compressing the spring a distance of 0.12m, the gun is able to launch a 20.g projectile to a maximum height of 20m when fired vertically from rest. Neglecting all resistive forces, A) determine the value of the spring constant and B) find the speed of the projectile as it moves through the rest position of the spring (where x=0).