It takes 2.75s for the rock to hit the water at 54.45mph, how high is the bridge?
54.45 mph = 79.86 ft/s
If the rock was thrown with an initial speed k, then
k-32*2.75 = -79.6
k = 8.4 ft/s
So, the rock was thrown upward at 8.4 ft/s from a height h:
h - 32*2.75 - 16*2.75^2 = 0
h = 209 ft