The standard form of a quadratic is given by:
ax^2 + bx + c
So for our function, we can say,
a = 0.2
b = -72
c = 17791
We can find the vertex (x-coordinate where minimum value occurs) by the formula -b/2a
So:
-(-72)/(2*0.2) = 180
Plugging this value into original function would give us the minimum (unit cost):
C(x) = 0.2x^2 - 72x + 17791
C(180) = 0.2(180^2) - 72*180 + 17791 = 11311