I've been given this problem to work on as homework, and I'm completely confused by the wording.

"A stone is thrown vertically upwards and after t seconds its height y metres above its starting point is given by y=100t-5y^2. Calculate the greatest height reached by the stone and the value of t when this happens."

I need to solve it using calculus but I'm not sure how. I've differentiated the equation given to get dy/dx=100-10t, though I don't know if that's the right thing to do. Help appreciated