Remark
You just need to put in 11/2 = 5.5 for t everywhere you see t. Why 5.5? Why not 11. The reason is that it takes the same amount of time to come down as it does to go up. If you have never taken a physics question, this one is a little unfair.
Givens
t = 5.5
Formula
f(t) = -16t^2 + 176t
Solve
f(5.5) = -16*(5.5)^2 + 176(5.5)
f(5.5) = -16*(30.25) + 968
f(5.5) = - 484 + 968
f(5.5) = 484 feet.