Answer: In this problem you want to rewrite the proportion 7:21 to 3:9.
First, a proportion is a relation between two quantities, this is if the proportion of A to B is 7:21, then you can represent it in a equation as:
7*A = 21*B
If you divide by the same number in both sides of the equation, you dont affect the equality, then if i divide both sides by 7, the equation turns into:
7*A/7 = 21*B/7
A = (21/7)*B = 3*B
Now if i multiply both sides by 3.
3*A = 3*(3*B) = 9*B
3A = 9*B
so now the proportion is 3:9.
Also, if you want to see if two proportions are equivalent, you need to see that quotients are the equal.
In this case 21/7 = 3, and 9/3 = 3.