When adding or subtracting fractions, they should have the same denominator. In this exercise, we have two fractions with different denominators.
7/12 - 5/8
To find the smallest common denominator we should rewrite the denominators of both fractions using only prime numbers.
12 &= 3* 2* 2
8 &= 2* 2* 2
Now, in order to get a common denominator we need to multiply each fraction by factors that appear only in the denominator of the other fraction. Therefore, we should multiply 712 by 2 and 58 by 3.