\(\Leftrightarrow\left(x+3\right)\left(2x-1\right)+\left(x-3\right)\left(x+3\right)=0\)
\(\Leftrightarrow\left(x+3\right)\left(2x-1+x-3\right)=0\)
\(\Leftrightarrow\left(x+3\right)\left(3x-4\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-3\\x=\dfrac{4}{3}\end{matrix}\right.\)