If $\displaystyle sin\;\beta = \frac{2}{7}$ find $\displaystyle cos\;2\beta$ without using a calculator.

First off I know that $\displaystyle cos\;2\beta = 1 - 2\;sin^2\;\beta$

But I'm having trouble plugging in the $\displaystyle sin\;\beta = \frac{2}{7}$

Anyone have any thoughts on this?