Answer:
15
Step-by-step explanation:
5/7 = x / 21
7 * 3 = 21
So we need to multiply the numerator by 3
5*3 = 15
The numerator is 15
5/7 = 15/21
multiply both 5 and 7 by 3 to get 15 and 21
5 x 3 = 15
_ _ _
7 x 3 = 21
