Final answer:
The rock will return to a height of 50 meters at t = 2.25 seconds.
Step-by-step explanation:
To find the time at which the rock returns to a height of 50 meters, we can set the height function h(t) equal to 50 and solve for t. The height function is given by h(t) = -4.9t^2 + 9t + 50.
So, we have the equation -4.9t^2 + 9t + 50 = 50. Simplifying this equation, we get -4.9t^2 + 9t = 0.
Rearranging, we have t(-4.9t + 9) = 0. This gives us two possible solutions: t = 0 and t = 9/4 = 2.25 seconds.
Since time cannot be negative, the rock returns to a height of 50 meters at t = 2.25 seconds.