$\left\{\begin{matrix}x=-4A^{-\frac{1}{2}}\sqrt{B}\text{; }x=4A^{-\frac{1}{2}}\sqrt{B}\text{, }&A\neq 0\text{ and }a=7\text{ and }b=400\\x\in \mathrm{C}\text{, }&B=0\text{ and }A=0\text{ and }a=7\text{ and }b=400\end{matrix}\right.$

$\left\{\begin{matrix}x=4\sqrt{\frac{B}{A}}\text{; }x=-4\sqrt{\frac{B}{A}}\text{, }&\left(B\geq 0\text{ or }A<0\right)\text{ and }\left(A>0\text{ or }B\leq 0\right)\text{ and }A\neq 0\text{ and }a=7\text{ and }b=400\\x\in \mathrm{R}\text{, }&B=0\text{ and }A=0\text{ and }a=7\text{ and }b=400\end{matrix}\right.$