平方完成

$3$\displaystyle{\begin{array}{lll}=a $ {x}^{2}+\frac{\hspace{2}b\hspace{2}}{\hspace{2}a\hspace{2}}x $ +c& \vspace{6}\\ =a{ $ x+\frac{\hspace{2}b\hspace{2}}{\hspace{2} 2a \hspace{2}} $ }^{2}- \frac{\hspace{2} {b}^{2} \hspace{2}}{\hspace{2} 4a \hspace{2}}+c& \vspace{6}\\ =a{ $ x+\frac{\hspace{2}b\hspace{2}}{\hspace{2} 2a \hspace{2}} $ }^{2}- \frac{\hspace{2} {b}^{2}- 4ac \hspace{2}}{\hspace{2} 4a \hspace{2}}& \vspace{6}\\ \end{array}}$