Find y” in terms of x and y by implicit differentiation (I.E. the second derivative. x^3+y^3=4^3 I thought the answer would be -128x/y^5. What am I

Find y” in terms of x and y by implicit differentiation (I.E. the second derivative.

x^3+y^3=4^3

I thought the answer would be -128x/y^5. What am I doing wrong?

P.s. See attached image for a copy of the problem.