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[tex]\it ax -a =12 \Leftrightarrow a(x-1) =12\Leftrightarrow a =\dfrac{12}{x-1}\ \ \ \ (*)
\\\;\\ \\
\it a = 3\ \ \ \ \ (**)
\\\;\\
(*), (**) \Rightarrow \it a = \dfrac{12}{3-1} =\ \dfrac{12}{2} \Longrightarrow a =6\ \ .[/tex]
