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i used the equation \[2as = V _{f}^{2} - V{i}^{2}\]

(or, you could start with v_i = 0, but watch the rock fall 55m

so, \[2 (-9.8)(-40) = Vf ^{2} - (17.1)^{2}\]

looks right so far

so vf would be 32.8?

yes, 55m is almost 4x the upward 15m (so v should be almost double)

so is 32.8 the answer? i was told to use the equation i listed

His eq'n is the same as yours, just re-arranged for v_f

Yep!
$$
V_f^2 = V_i^2 + 2gs=0-2\times9.81\times55\\
\implies V_f=32.8~m/s
$$

..... negative?

Just indicates direction, downward. Up is positive

the same direction as the acceleration.

it says it's wrong

ooh, they asked for _speed_ (how fast) not velocity :-(

speed is magnitude, velocity includes directional information. So speed would just be 32.8 m/s