Why is vy negative and not positive because i solved it and it resulted in a positive number?

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.