When two lines cross, they form two pairs of vertical angles. Vertical angles are across the intersection point from each other.

Vertical angles always have equal measure. We can see this because they are always supplementary with the same angle. For example:

This is always true!

$a+b = 180$ so $a = 180-b$.

$c+b = 180$ so $c = 180-b$.

That means $a = c$.