\(sinx=-1\Rightarrow x=-\dfrac{\pi}{2}+k2\pi\)
\(\Rightarrow0\le-\dfrac{\pi}{2}+k2\pi\le4\pi\)
\(\Rightarrow\dfrac{1}{4}\le k\le\dfrac{9}{4}\)
\(\Rightarrow k=\left\{1;2\right\}\)
\(\Rightarrow x=\left\{\dfrac{3\pi}{2};\dfrac{7\pi}{2}\right\}\) \(\Rightarrow\dfrac{3\pi}{2}+\dfrac{7\pi}{2}=5\pi\)