`d) ( x - 3 )/4 = 9/( x - 3 )`
`⇔ ( x - 3 ) . ( x - 3 ) = 9 . 4`
`⇔ ( x - 3 )^2 = 36`
`⇔` $\left[\begin{matrix}( x - 3 )^2 = 6^2\\( x - 3 )^2 = ( - 6 )^2\end{matrix}\right.$
`⇔` $\left[\begin{matrix}x - 3 = 6\\x - 3 = - 6\end{matrix}\right.$
`⇔` $\left[\begin{matrix}x = 9\\x = - 3\end{matrix}\right.$
Vậy `, x ∈ { 9 ; - 3 } .`