б) $$\frac{1}{3x+y} - \frac{1}{3x-y} = \frac{(3x-y) - (3x+y)}{(3x+y)(3x-y)} = \frac{3x-y-3x-y}{9x^2-y^2} = \frac{-2y}{9x^2-y^2} = -\frac{2y}{9x^2-y^2}$$ Ответ: $$\frac{-2y}{9x^2-y^2}$$
в) $$\frac{4-3b}{b^2-2b} + \frac{3}{b-2} = \frac{4-3b}{b(b-2)} + \frac{3}{b-2} = \frac{4-3b+3b}{b(b-2)} = \frac{4}{b(b-2)}$$ Ответ: $$\frac{4}{b(b-2)}$$