$$ \operatorname{Var}\left(x_{t}\right)=\operatorname{Var}\left(\mu+\varepsilon_{t}-\theta_{1} \varepsilon_{t-1}-\theta_{2} \varepsilon_{t-2}-\cdots-\theta_{q} \varepsilon_{t-q}\right)=\left(1+\theta_{1}^{2}+\cdots+\theta_{q}^{2}\right) \sigma_{\epsilon}^{2} $$