See picture.
Why have you not attempted anything yourself? You are told that $\displaystyle u= \frac{2(m+ n)}{n}$ and $\displaystyle v= \frac{4m}{(m+n)^2}$, the two legs of the triangle. Do you know the Pythagorean theorem: $\displaystyle w^2= u^2+ v^2$? What are $\displaystyle u^2$ and $\displaystyle v^2$?