`ĐKXĐ:`
`x² + 2x - 3 ne 0`
`<=> x² - x + 3x - 3 ne 0`
`<=> x(x - 1) + 3(x - 1) ne 0`
`<=> (x + 3)(x - 1) ne 0`
`<=>` \(\left[ \begin{array}{l}x + 3 \ne 0\\x - 1 \ne 0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x \ne -3\\x \ne 1\end{array} \right.\)
`=> D = RR` \ `{-3; 1}`