Those aren't fractions, so there's nothing to find for the LCD (least common denominator).
Or, you could say "what is the LCD of (x+18)/1 and (x-18)/1 ?" Then the LCD would be 1.
If you mean "what is the LCM (least common multiple) of x+18 and x-18 ?" then it's (x+18)*(x-18) = x^2 - 324