3) \(x=\left\{1;2;3;6\right\}\)
4) \(\Leftrightarrow15+\left(x+2\right)^2=54\\ \Leftrightarrow\left(x+2\right)^2=36\\ \Leftrightarrow\left[{}\begin{matrix}x+2=-6\\x+2=6\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=-8\\x=4\end{matrix}\right.\)
