Ball kicked with initial velocity of 15 m/s at 60* above the horizontal. Assuming air resistance is negligible, only influence on ball after been kicked is the acceleration due to gravity (g=-9.81 m/s2).

Draw a triangle. We are looking for the adjacent leg of the triangle. We know the angle and length of the hypotenuse. Use SOH CAH TOA to solve from there. We use cosine because it's Adjacent/Hypotenuse.