hello, i am trying to evaluate a trig ratio but the example is quite hard to follow. can someone please explain how this question is done,

given that $\displaystyle sinx=\frac{12}{13}, $and that $\displaystyle \frac{\pi}{2}< x<\pi$

show that$\displaystyle cos x= \frac{-5}{13}$

hence without calculators evaluate $\displaystyle sin (x +\pi)$ and $\displaystyle cos2x$