ini gimana ya tolong bantu

[tex]\displaystyle \text{biaya produksi }x\text{ barang adalah}\\c(x)=xb(x)\\c(x)=x\left(\frac23x^2-4x-24+\frac{155}{x}\right)\\c(x)=\frac23x^3-4x^2-24x+155\\\\\text{agar biaya minimum, maka }\\c'(x)=0\\2x^2-8x-24=0\\x^2-4x-12=0\\(x-6)(x+2)=0\\x=6\vee x=-2\\\\c(6)=\frac23\cdot6^3-4(6)^2-24(6)+155\\c(6)=144-144-144+155\\\boxed{\boxed{c(6)=11}}[/tex]