$a)_{}$ $x^2+3x-4=0_{}$
$⇔x^2+4x-x-4=0_{}$
$⇔x.(x+4)-(x+4)=0_{}$
$⇔(x+4)(x-1)=0_{}$
$⇔_{}$ \(\left[ \begin{array}{l}x+4=0\\x-1=0\end{array} \right.\) $⇔_{}$ \(\left[ \begin{array}{l}x=-4\\x=1\end{array} \right.\)
$b)_{}$ $x^3-7x^2+x-7=0_{}$
$⇔x^2.(x-7)+(x-7)=0_{}$
$⇔(x^2+1)(x-7)=0_{}$
$⇔_{}$ \(\left[ \begin{array}{l}x^2+1=0\\x-7=0\end{array} \right.\) $⇔_{}$ \(\left[ \begin{array}{l}x=2(vôlý)\\x=7\end{array} \right.\)
$Vậy_{}$ $x_{}=7$
