[tex]\bf x^2-ax+bx-ab=0\implies \stackrel{\textit{grouping first}}{(x^2-ax)+(bx-ab)}=0 \\\\\\ \stackrel{\textit{common factor}}{x\underline{(x-a)}+b\underline{(x-a)}}=0\implies \stackrel{\textit{common factor}}{\underline{(x-a)}(x+b)}=0\implies \begin{cases} x-a=0\\ \boxed{x=a}\\ \cline{1-1} x+b=0\\ \boxed{x=-b} \end{cases}[/tex]
