\(\dfrac{z^8+11z^4+30}{18z^2+12z+2}\cdot\dfrac{24z^2+8z}{z^2+6z}\)
\(=\dfrac{\left(z^4+6\right)\left(z^4+5\right)}{2\left(9z^2+6z+1\right)}\cdot\dfrac{8z\left(3z+1\right)}{z\left(z+6\right)}\)
\(=\dfrac{\left(z^4+6\right)\left(z^4+5\right)}{\left(3z+1\right)^2}\cdot\dfrac{4\left(3z+1\right)}{z+6}\)
\(=\dfrac{\left(z^4+6\right)\left(z^4+5\right)\cdot4}{\left(3z+1\right)\left(z+6\right)}\)