I got a negative answer am i wrong?

Question

The number of mosquitoes M(x), in millions, in a certain area depends on the June rainfall X, in inches, according to the equation M(x)=16x-x^2. what rainfall produces the maximum number of mosquitoes? 

Answer 1

This model is a quadratic equation that opens downwardly because a<0.

Therefore the maximum occurs at the vertex.

The formula for the x coordinate of the vertex is x_vertex = -b/(2a) and if you substitute a=-1 and b = 16 you will get a POSITIVE x value.  Then M(x) is the resultant # of mosquitoes.