a: \(=\dfrac{24x^3}{5y^2z^4}\cdot\dfrac{15y^3z^2}{8x^2}=\dfrac{9xy}{z^2}\)
b: \(=\dfrac{\left(x-5\right)\left(x+5\right)}{x\left(x-3\right)}\cdot\dfrac{\left(x-3\right)\left(x+3\right)}{x\left(x+5\right)}=\dfrac{\left(x-5\right)\left(x+3\right)}{x^2}\)