The two rectangles have 9xy^3 and 12 x^2 y^4
Rewriting 9xy^3 = 3*3 *x*y*y*y
12 x^2 y^4 = 2*2*3* x*x*y*y*y*y
The common factors are 3 *x*y*y*y = 3 xy^3
The width is 3xy^3
9xy^3 is 3xy^3 *3 so what remains is 3 after the GCF is removed
That would be the length of the first rectangle
12x^2y^4 = 3x y^3 ( 4xy) so what remains is 4xy
That would be the length of the second rectangle