Remember that this type of problem can be solved by applying the conservation of the linear amount of movement.
 1: cannonball
 2: boulder
 m1v1i + m2v2i = m1v1f + m2v2f
 v1f = v2f = vf (they fall together)
 However, in this case we have:
 "cannonball is fired into a stationary 500kg boulder"
 Thus
 v2i = 0
 answer:
 the initial speed of the boulder was 0 m / s (stationary 500kg boulder)