Question 1155427
The derivative is 
12*(2/3)*x^(-1/3)-1 and set that equal to 0.
8x^(-1/3)-1=0
8/x^(1/3)=1
x^(1/3)=8
x=512
at that point, y=12*64-512=256

{{{graph(300,300,-200,1728,-80,300,12x^(2/3)-x)}}}