if it takes 21 yards to make 7 quilts, then we can divide the reactants by the product, so we just do:
21 / 7 = 3
Since it takes 3 yards to make 1 quilt, it will take 21 yards to make 7 quilts. Now we've figured out how many yards it takes per single quilt, but that'ts not what the question asks, so we go back to the question and see that we need enough material to make 12 quilts. From here, the solution is as easy to find as saying
3 * 12 = 36
36 yards.
Now we know how many yards it takes to make 12 quilts and the problem is solved.