Given that $\displaystyle \sin u=\frac{7}{25}$ and $\displaystyle \tan u<0$

Find the exact value of $\displaystyle \cos u,

\sin 2u,

\cos 2u,

\tan 2u,

$

I was absent this day in class and am not sure about where to start with this problem. I obviously know that you won't do them all, and I don't expect you to or else I'd never learn! But guidance on how to do find $\displaystyle \cos u$ and possibly one of the double angle ones would be greatly appreciated.