i dont know hot to get common denominator

The simplest way to find a common denominator is to "skip count" (count by multiples) by each of the numbers in question. For example, 9, 18, 27, 36, 45, 54, 63, 72 . . . and 12, 24, 36, 48, 60, 72, 84, . . . . Any numbers on both lists are "common denominators" (36 and 72); the first such number on both lists is the Lowest Common Denominator (LCD).

A more complicated way involves using the prime factorizations of the numbers in question. The p. f. for 9 = 3 x 3; the p. f. for 12 = 2 x 2 x 3. Next, line the two p.f.'s up like this, showing where any factors from the p.f.'s overlap:

2 x 2 x **3**

**3** x 3

Now, write the factors out in one line, using all the factors from the 2 lines, but where they overlap, write only one of the 2 overlapping factors:

2 x 2 x **3** x 3 = 36 (the LCD)