The rock hits the surface when the height is 0, or when
t^2-8t-10=0
Use the quadratic formula: t = -1.099 and 9.099. Time can't be negative, so the answer is t = 9.099.
This problem is solved also by this person: https://web2.0calc.com/questions/equation-math-question-pls-help-gt-gt-gt-anyone
Pls do not repeat questions if it already solved!!!