` a) ` ` \frac{-25x^{2}y^{5}}{3x} : 15xy^2 `
` = \frac{-25x^{2}y^{5}}{3x} . \frac{1}{15xy^2} `
` = \frac{-5xy^{3}}{3x} . 1/3 `
` = \frac{-5y^{3}}{9} `
` b) ` ` \frac{x^2 - 25}{x^2 - 3x} : \frac{x^2 + 5x}{x^2 - 9} `
` = \frac{(x - 5)(x + 5)}{x(x - 3)} . \frac{(x - 3)(x + 3)}{x(x + 5)} `
` = \frac{x - 5}{x} . \frac{x + 3}{x} `
` = \frac{(x - 5)(x + 3)}{x^2} `
` = \frac{x^2 - 2x - 15}{x^2} `