Find the number of units x that produces the minimum average

Find the number of units x that produces the minimum average cost per unit C in the given equation.

C = 0.001x³ + 9x + 432

x =  units

Solution

Average cost per unit

C(x)/x =.001x^2+9+432x^-1
d/dx[C(x)/x]=.002x-432/x^2=0
.002x=432/x^2
.002x^3=432
x^3=432000/2
x=60