If x^2 + x + y^3 + y =1 then find y". Your answer must not contain y'. Do not simplify at all.

So I found the first derivative because I dont understand why the answer cannot contain y' if you have to find y" :S but i found the first derivative and got:

2x + 1/ 3y^2 = dy/dt

then my second deriv was y"= 6y(y')(2x-1) - (2)(3y^2)/ (3y^2)^2

I got 1.5/3 and I dont understand why, any help would be greatly appreciated