Prove that, for the $M / G / 1$ queueing system,
$$
\begin{aligned}
\sigma_N^2= & \frac{\lambda^3 E\left[s^3\right]}{3(1-\rho)}+\left(\frac{\lambda^2 E\left[s^2\right]}{2(1-\rho)}\right)^2 \\
& +\frac{\lambda^2(3-2 \rho) E\left[s^2\right]}{2(1-\rho)}+\rho(1-\rho)
\end{aligned}
$$