Well, honey, the least common denominator for 7 and 9 is 63. You gotta find the smallest number that both 7 and 9 can divide into evenly, and that's 63. So there you have it, 63 is the magic number for those two divas.
There is none because the Least Common Denominator (or LCD) is the Least Common Multiple (LCM) of the denominators of two or more fractions. Even if you converted your numbers (x) to their fraction equivalents (x/1), the LCD will always be 1.