Set H = 0 and solve for t. Here's why: Your ball is at height = 0 TWICE, at the start and at the end of its journey through the air (when you launch and when it lands.) Since the time up = time down, you can find the second solution for t (the first solution for t is zero, the time of launch) to find out how long it took to land... AND if you divide that total time by 2, you will know how much time it took for the ball to reach the top of its arc (when velocity became zero.) Now solve the problem again. Take that total time divided by two and substitute it for t in the equation. You are going to solve the equation to find H, the maximum height of the ball.