a: \(=\dfrac{-\left(a-3\right)\left(a^2+3a+9\right)}{5\left(a+2\right)}\cdot\dfrac{3\left(a+2\right)}{a-3}\)
\(=\dfrac{-3\left(a^2+3a+9\right)}{5}\)
b: \(=\dfrac{\left(m-2\right)\left(m-3\right)}{\left(m+3\right)\left(m+4\right)}\cdot\dfrac{m+4}{\left(m-3\right)^2}\)
\(=\dfrac{\left(m-2\right)}{\left(m+3\right)\left(m-3\right)}\)