Write down the first three terms in the binomial expansion of $\displaystyle \sqrt[4]{1+\frac{x}{81}}$

Here i got

$\displaystyle 1+\frac{x}{324}-\frac{x^2}{69984}$

Then deduce that $\displaystyle \sqrt[4]{80}=2.9907$ .. not sure with this part

That should be the approximation sign , i am not sure how to do it with latex .