If w=width, then length=w+12, so area in square inches=88=w(w+12).
w^2+12w=88; w^2+12w+36=88+36 (completing the square);(w+6)^2=124; w+6=+sqrt(124)=+2sqrt(31)=+11.14.
So w=11.14-6=5.14" and length=17.14" (the other solution is negative, so can be ignored).