Taking the equation as given:

If you have calculus, the derivative is -20t + 260 = 0 at the maximum height, i.e. at t =13 sec.

If you do not have calculus, you need to know that the vertex of a parabola occurs at -b/2a where the parabola is :

at

^{2}+ bt + c, which for this problem is -260/-20 = 13 sec.

Substitute 13 into the original equation to get the maximum height of 1690 feet.