p(z) = p(x, y) Unknown = p(y|x) Known × p(x) Known From the product rule of probability, \begin{align*} p(\mathbf z) = \highlightcap<2->[blue]{p(\mathbf x, \mathbf y)}{Unknown} = \highlightcap<2->[red]{p(\mathbf y | \mathbf x)}{Known} \times \highlightcap<2->[red]{p(\mathbf x)}{Known} \end{align*} 3/4