using the implicit differentiation, find y' (or dy/dx) of y^4 - 3xy + x = y^3

4y^3 - 3y - 3x + 1 = 3y^2

dy/dx(4y^3 - 3y - 3x) + 1 = 3y^2

dy/dx(4y^3 - 3y - 3x) = 3y^2 - 1

dy/dx = (3y^2 - 1)/(4y^3 - 3y - 3x)

what did i do wrong? =/

the answer is supposed to be (3y - 1)/(4y^3 - 3y^2 - 3x)