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I was wondering if someone could explain how to tell whether V = - or +?

I'm getting answers right using the formula

V^2 = sqrt(Vo^2 + 2ay)

but I'm getting positive answers. How do you tell whether the velocity is in the + or - direction?

Sample Question:

A hot-air balloon is rising upward with a constant speed of 2.50m/s. When the balloon is 3.00m above the ground, the balloonist accidentally drops a compass over the side of the balloon. How much time elapses before the compass hits the ground?

I got V = 8.1m/s but the right answer requires using -8.1m/s.

thanks for any help :)