Let x represent the cost of 1 yard.
We were told that 3 whole and 1/2 yards of fabric cost 21.00.This can also be written as 3.5 yards cost 21.00. Thus, we have the following equations
3.5 = 21
1 = x
By cross multiplying, it becomes
3.5 * x = 21 * 1
3.5x = 21
x = 21/3.5
x = 6
The cost of 1 yard is 6.00