# Transfer function: from poles and zeros to polynomial coefficients

I understand how this transfer function was solved except I don't know how to get the 0.87. Any help would be appreciated.

In short: $$(0.937)^2=0.878$$
\begin{align}(z-re^{j\theta})(z-re^{-j\theta})&=z^2-r\left(e^{j\theta}+e^{-j\theta}\right)z+r^2\\&=z^2-2r\cos(\theta)z+r^2\end{align}