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**Help please! simple Physics motion in one dimension problem! Urgent!?**

A rock is shot vertically upward from the edge of the top of a tall building. The rock reaches its maximum height above the top of the building 1.60 s after being shot. Then, after barely missing the edge of the building as it falls downward, the rock strikes the ground 6.00 s after it is launched. In SI units: (a) with what upward velocity is the rock shot, (b) what maximum height above the top of the building is reached by the rock, and (c) how tall is the building?

(a) and (b) i know how to get the answer. but im stuck on c.

Here's what i did:

I chose up as my negative direction (-9.8m/s^2) and down as my positive direction (+9.8m/s^2).

i found the initial velocity to be 15.68m/s

so to find the height of the building i use the formula 'distance = Vo(t) + 1/2at^2'

height of building (distance) = 15.68 (6) + 1/2 (+9.8)(6^2) = 270.48m

my answer is wrong! its suppose to be 82.3m, if i had chose up as my positive direction (+9.8m/s^2) and down as my negative direction (-9.8m/s^2), i would have gotten the right answer. BUT i thought if you stayed consistent with your pos and neg directions you will get the right answer either way but im not getting it.

HELP PLEASE!!!!!!