Write it as , then .

You know that when x = 2, [tex]\displaystyle \frac{dy}{dx} = 0[/math] due to the function being minimised there. You also know that when x = -2, y = 41, and when x = 5, y = 20. Substitute all this information into the equations so that you get equations in terms of a,b,c to solve simultaneously. See how you go.