C(x)=10000+5x+1/9x^2.
C(x) is a quadratic function (parabolic shape), so the minimum value corresponds to the x-value of the vertex's point.
We find vertex using the fact that the vertex has coordinate x=−b/2a
On this case b = 5
and a =1/9
So, x = -45/2 = −22.5
