\(\Leftrightarrow3\left(x+5\right)=x\left(x+5\right)\)
\(\Leftrightarrow x\left(x+5\right)-3\left(x+5\right)=0\)
\(\Leftrightarrow\left(x-3\right)\left(x+5\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x-3=0\\x+5=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=3\\x=-5\end{matrix}\right.\)