I really need help with this. it's projectile motion i think?

The COS of 55 Degrees is .5735. So (1-.5735) * 100 is the percentage of the initial velocity that is horizontal. Therefore 42.64 percent of the initial velocity should remain constant in a horizontal direction throughout the flight of the stone (uming no air resistance). 18m/s * .4264 = 7.675 m/s.