Not sure how to find speed of a falling object! Can someone please tell me the formula!?

A 4.00 kg cannon ball is taken to the top of a building and is held 40.0 m above the ground. It is tossed downward with a speed of 8.00 m/s. Using energy conservation and uming negligible air resistance, calculate its speed when it has already fallen 30.0 m. Show all work.