$$( 2 x + 3 ) ^ { 2 } = a ( x + 3 ) ^ { 2 } + b x + c$$
$\left\{\begin{matrix}a=-\frac{-4x^{2}+bx-12x+c-9}{\left(x+3\right)^{2}}\text{, }&x\neq -3\\a\in \mathrm{C}\text{, }&b=\frac{c}{3}-3\text{ and }x=-3\end{matrix}\right.$
$\left\{\begin{matrix}b=\frac{9-9a-c+12x-6ax+4x^{2}-ax^{2}}{x}\text{, }&x\neq 0\\b\in \mathrm{C}\text{, }&a=-\frac{c}{9}+1\text{ and }x=0\end{matrix}\right.$
$\left\{\begin{matrix}a=-\frac{-4x^{2}+bx-12x+c-9}{\left(x+3\right)^{2}}\text{, }&x\neq -3\\a\in \mathrm{R}\text{, }&b=\frac{c}{3}-3\text{ and }x=-3\end{matrix}\right.$
$\left\{\begin{matrix}b=\frac{9-9a-c+12x-6ax+4x^{2}-ax^{2}}{x}\text{, }&x\neq 0\\b\in \mathrm{R}\text{, }&a=-\frac{c}{9}+1\text{ and }x=0\end{matrix}\right.$