7/12=21/36; 5/9=20/36.
To get the numerators: 21=7×(36/12) and 20=5×(36/9).
7 = 3×7
12 3×12
5 = 4×5
9 4×9
We needed to multiply 12 by 3 to get 36; and we needed to multiply 9 by 4 to get 36; so we multiply the numerator by the same number and we get fractions with the same (common) denominator.