Angle between two intersecting lines

Let $l$ be the line that is perpendicular to the $xy$ -plane and passes through the point $(2, 1, -3).$ Let $m$ be the line that is perpendicular to the $x$ -axis and passes through the same point $(2, 1, -3).$ Find the value of $\cos {\theta} ,$ where $\theta$ is the acute angle between $l$ and $m.$