a: \(\dfrac{3}{x^2+3x}-\dfrac{x+9}{x^2-9}\)
\(=\dfrac{3}{x\left(x+3\right)}-\dfrac{x+9}{\left(x-3\right)\left(x+3\right)}\)
\(=\dfrac{3x-9-x^2-9x}{x\left(x-3\right)\left(x+3\right)}=\dfrac{-x^2-6x-9}{x\left(x-3\right)\left(x+3\right)}=\dfrac{-x-3}{x\left(x-3\right)}\)
b: \(=18x^3y^3-9xy-xy^2\)