Help please I don't understand this?

A 70.0-kg circus performer is fired from a cannon that is elevated at an angle of 40.0 degrees above the horizontal. The cannon uses strong elastic bands to propel the performer, much in the same way that a slingshot fires a stone. Setting up for this stunt involves stretching the bands by 3.00 m from their unstrained length. At the point where the performer flies free of the bands, his height above the floor is the same as that of the net into which he is shot. He takes 4.00 s to travel the horizontal distance of 50.0 m between this point and the net. Ignore friction and air resistance and determine the effective spring constant of the firing mechanism.