Answer:
1:3
Explanation:
Since 7 goes into both 21 and 63 you are going to divide 21 and 63 by 7:
21/7 = 3
63/7 = 9
So now we have 3:9 but we can go even further than that, 3 can go into both 3 and 9 so we are going to divide 3 and 9 by 3:
3/3 = 1
9/3 = 3
So we now have 1:3 ratio for the simplest form of of the ratio 21:63
Hope this helps you!!!