Because the width is 3/7 of 21, we can multiply 3/7 by 21 to find the width.  3/7 * 21 = 9.  So, the width is 9.
Now we know that the width is 9 and the length is 21.  From this, we can find the perimeter and area of the rectangle.  The perimeter is equal to 9 + 9 + 21 + 21 = 60.  So, the perimeter is 60 inches.
Now, we need to find the area.  We do this by multiplying 21 * 9.  This is equal to 189, so the area is 189 square inches.