Stone Throw, Found answer but don't understand how ?

[Monocerotis]

Homework Statement

A stone is thrown from a bridge with an initial vertical velocity of magnitude 4.0m/s/ Determine the stone's velocity after 2.2s if the direction of the initial velocity is (a) upward, and (b) downward. Neglect air resistance.

Homework Equations

A_av=V_f - V_i / Δt

V_i + A_av(Δt) = V_f

The Attempt at a Solution

b) 2.6 x 10¹m/s
Easy to find as you enter in the given information into the formula.

(a) on the other hand I have difficulty understanding. I attempted the problem, with this formula
(-V_i) + A_av(Δt) = V_f. The answer I got corresponds to the answer in text, but I don't really understand how that works. I'd like to know for similar problems in the future.

 

[willem2]

You use the same equation to solve both problems, but if v_i is positive in b, it must be negative in problem a.

 

[Monocerotis]

You use the same equation to solve both problems, but if v_i is positive in b, it must be negative in problem a.

Yes that's apparently obvious, I want to understand why however. The "theory" behind it.