分数式の計算

問題次の計算をせよ。$${\small (1)}~\frac{36x^2y}{9xy^3}$$$${\small (2)}~\frac{2y^3}{3x^2}\div \frac{4y^2}{6x}$$$${\small (3)}~\frac{x^2-6x-7}{x^2-1}$$$${\small (4)}~\frac{x^2-5x+6}{x^2-2x-3}$$