Step-by-step explanation: The rational expression in this case is undefined if the the denominator is equal to zero, which means that the x cannot be whatever makes the denominator zero, so:
x² - 81 = 0 ⇒ x² = 81 ⇒ x = +/- √81 ⇒ x = +/- 9
Note that we are using the equal sign just because this particular problem askes for a solutions that is undefined.
