The question asked for the area in square *yards*, so we have to be careful to convert the dimensions of the floor to yards before measuring the area. There are 3 feet in 1 yard, so 21 ft = 21/3 = 7 yd, and 9 ft = 9/3 = 3 yd.
To the area of any rectangle is simply the product of its length and width; given a length of 7 yd and a width of 3 yd, the area of this floor is 7 x 3 = 21 sq. yd
