Step-by-step explanation:
You are given the initial velocity, the displacement, and the acceleration. You're looking for the final velocity. So you use the equation:
v² = v₀² + 2aΔy
When you solve for v, you take the square root. Your calculator will return a positive answer, but there are actually two possible answers: positive and negative.
v = ±√(v₀² + 2aΔy)
You must use the physical context of the problem. If we take up to be the positive direction, then v must be negative, since the projectile is moving down.