In order to sum up any two fractions, you need both fractions to have the same denominator.
The denominator is the number under the fractional bar. The denominator of the first fraction is 7. The denominator of the second fraction is what ? It's 3.
7 is different from 3 : 7 ≠ 3
So in order to sum these two numbers, we need their denominator to be the same. How ?
A solution that always work, is simply to multiply each other : 7 * 3