Hello, DreadfulGlory!

A projectile is thrown upward so that its height above the ground after t seconds is given by the function:

. .

After how many seconds does the projectile take to reach its maximum height?

I'll assume that you are not familiar with Calculus.

The graph of is a down-opening parabola.

Its maximum is at its.vertex

The formula for the vertex is: .

We have: .

Hence: .

The projectile takes 18 second to reach maximum height.

[The maximum height is: . units.]