The m,maximum height is the vertex. The (x, y)
x is the time
y is the height
h= -5t2 + 21t
find the vertex
-b/2a is the x
-21/2(-5) = -2.1 (-10) = -2.1 is that man seconds its at its highest
now plug into
h= -5(2.1)^2 + 21(2.1) the answer is the max height