The answer is 30 11/12 (almost 31) square feet.

Solution 3 1/2 ft. x 8 5/6 ft

7/2 x 53/6 = 371/12 which is about 31

Find the GCF of 12 and 18:  Write the Prime Factorization of both numbers 12=2x2x3 & 18=2x3x3

Draw the two intersecting circles for the Venn Diagram.  Place the common prime factors in the intersection [in this case the 2 & 3] and there you have it!! 2x3 = 6 

What remains in the other two sections is the simplified  form of the fraction 12/18.  2/3

If there are NO common prime factors the GCF is 1.