This question wants you to find a common denominator for the fractions.
This means finding the LCM, least common multiple, for 21 and 9.
This can be done by listing the multiples for each number until they meet at a common one.
9:
9
18
27
36
45
54
63
21:
21
42
63
This means the LCM of 21 and 9 is 63.
So the lowest possible common denominator is 63.
21 • 3 = 63
So you have to multiply the numerator of 2/21 by 3 as well.
2 • 3 = 6
2/21 = 6/63
Now do the same for 1/9.
9 • 7 = 63
Multiply the numerator, 1, by 7.
1 • 7 = 7
1/9 = 7/63
So in the first blanks, you would put 6/63 for what 2/21 is equal to and 7/63 for what 1/9 is equal to.
7/63 is greater than 6/63.
7/63 > 6/63
That means 1/9 > 2/21
So 2/21 < 1/9 is the answer to the last blank.
Hope this helps!