The area of the rectangle is 21 in2. the length of the rectangle is 6in. what is the width of the rectangle?

Area of a Rectangle is Length times Width. So it's basically sort of like a simple multiplication/division problem. So it's basically What times 6 equals 21 (assuming the 2 was just placed by accident). So:

21 divided by 6
6 times 3 equals 18
21-18=3
3 with a remainder of 3

The remainder is out of 6, so your answer is 3 and 3/6 or 3 and 1/2.

Hope that helps! =)