If a stone is thrown vertically upward with a velocity of 10 m/s, its height in meters after t seconds is h = 10t - 0.83t^2. What is the velocity of the stone after it has risen 25 meters?

I am not sure how to do this problem. I would guess it involves finding f'(h) = 10 - 1.66t, but please show me how to find the answer in a way that makes sense.