2−18m21+3m\frac{2 - 18m^2}{1 + 3m}1+3m2−18m2 = 2(1−9)m21+3m\frac{2(1 - 9)m^2}{1 + 3m}1+3m2(1−9)m2
= 2(1+3m)(1−3m)1+3m\frac{2(1 + 3m)(1 - 3m)}{1 + 3m}1+3m2(1+3m)(1−3m)
= 2(1−3m)2(1 - 3m)2(1−3m)