1)A ball is thrown upward with an initial velocity of v = 20 m/s. How long is the ball in the air? What is the greatest height of the ball? When is the ball 15 m above the ground? 1) I have figured out t total = 4.08s greatest height = 20.4m For the last bit (Finding the time when x=15m), I am having trouble. I know the answer but cannot work it through. Do I use s = ut+1/2at^2 for it? If I do, it wont work out because the 20.4 m has to come into it, doesnt it? Please help!