Answer:
t = 4.875 seconds
Step-by-step explanation:
The vertex of the quadratic function ...
ax² +bx +c
is found at ...
x = -b/(2a)
Your quadratic function has a=-16, b=156, so the highest point is at ...
t = -156/(2(-16)) = 39/8 = 4.875
The rock reaches its maximum height at t=4.875 seconds.
