Question 874241
Plug in f(t) = 0. We do this because f(t) represents the height and a height of 0 means you're at the ground.


f(t) =-5t^2 + 20t + 60


0 = -5t^2 + 20t + 60


-5t^2 + 20t + 60 = 0


Now solve for t using the quadratic formula


*[invoke quadratic_formula -5,20,60, "t"]


Ignore the negative t value (negative time values make no sense)


The only practical solution is {{{t = 6}}}


So it takes 6 seconds for the object to hit the ground.