You're asking in the ap section, so i'll give the ap calculus-style answer. alternatively, you can find the vertex of the parabola and use the "y"-coordinate as the answer.
the maximum height can occur at a critical number, that is, a number such that .
the derivative of can be calculated as follows:
find a value of that is a zero of the derivative:
this is parabola that is opening downward; we can assume there is a maximum at .
to find that maximum, calculate .