б) (3c+9d)²; c+3d

б) $$ \frac{(3c + 9d)^2}{c + 3d} = \frac{(3(c + 3d))^2}{c + 3d} = \frac{9(c + 3d)^2}{c + 3d} = 9(c + 3d) $$

Ответ: $$9(c+3d)$$