If x^2/3 = y, what does x^4 equal in terms of y?

You need to calculate the value that, when multiplied by 2/3, will result in 4.

4 ÷ 2/3  = 4 x 3/2 = 12/2 = 6

So:  (x^2/3)⁶ = x⁴

Since y = x^2/3  --->   y⁶  =  (x^2/3)⁶  =  x⁴