How do I Find y' for y= y(x) defined implicitly by 3xy - x^2 - 4=0?

3xy - x^2 - 4 = 0
3xy = x^2 + 4
y = (x^2 + 4) / (3x)
y = (1/3)x + (4/3)x^(-1)
y' = (1/3) - (4/3)x^(-2)
y' = (x^2 - 4) / (3x^2)