I'm dealing with the expression $x = \frac{1}{3}y(y+1)(2y+1)^2(2y^2+2y+1)$. What is this approximately, if one is explicitly writing y in terms of x? There's no general formula for sixth powers unfortunately. 

Also can one given an approximation of this so that the difference between the true y and the approximation go to zero? (Not just the ratio).