word problem involving the maximum or minimum of a quadratic function

Question

A supply company manufactures copy machines. The unit cost C (the cost in dollars to make each copy machine) depends on the number of machines made. If x machines are made, then the unit cost is given by the function 

C(x) = 0.4x^2 - 112x + 15,045. How many machines must be made to minimize the unit cost? 

Number of copy machines: ____
Thanks!! 

Answer 1

In f(x) = ax² + bx + c, the max/min occurs at x= -b/2a.

For the given function what is that value?