Agolfer hits the golf ball. the quadratic function y = −16x^2 + 80x gives the time x seconds when the golf ball is at height 0 feet. how long does it take for the golf ball to return to the ground? !

You need to calculate the two zeros of the function. The first zero is the "start", the second zero is the "end" of the trajectory.

Well...

0 = x(-16x+80)

First zero is x=0 (the start of trajectory)

Second zero:

-16x+80=0 x=5! (The end of the trajectory!)

Good luck!
M.