The maximum number of mosquitoes would exist if 7.5 inches of rain fell.
We can find that by looking for the maximum value in the equation. This value always falls at the vertex. The x-value of the vertex (which is what we are looking for) can always be found through the equation -b/2a, in which a is equal to the coefficient of x^2 (-1) and b is equal to the coefficient of x (15).
-b/2a
-(15)/2(-1)
-15/-2
7.5
