Answer :
To find out the answer to this, you need to find the greatest common factor of 21 and 36. To find that, make factor trees of both numbers and multiply the numbers that appear in both of them together.
21's factors are: 3 and 7
36's factors are: 3, 3, 2, and 2
They have only one 3 in common, so the GCF is 3, and therefore the largest side length of the squares.
21's factors are: 3 and 7
36's factors are: 3, 3, 2, and 2
They have only one 3 in common, so the GCF is 3, and therefore the largest side length of the squares.