If the long side on both is 9ft and 3ft then 3•3 is 9 so for the short side you do the same. If the short side is 1ft and the other is "j" then it's 1•3 is 3.
1•3=3
Sorry if it's confusing the way I explained it.
Judging by the question I noticed that the two both share x^3 in common. You can divide x^4 by x^3 and get x as a result, and x^3 can still divide into itself.
Thus your answer should be C, x^3
(12+7x)(12-7x)
Because the square root of 144 is 12 and for 49 is 7