ada yang tau gimana caranya??? mohon bantuannya

[tex]P(x) = \frac{1}{3}x^2-12x+150[/tex]
[tex]P'(x) = \frac{2}{3}x-12 [/tex]
[tex]0 = \frac{2}{3}x-12 [/tex]
[tex]x = 18 [/tex]
[tex]P(18) = \frac{1}{3}18^2 - 12(18) + 150[/tex]
[tex]P(18) = 42[/tex]
Maka Biaya minimum Rp 42.000.000