## Geometry: Common Core (15th Edition)

$x = 30$
We can treat this diagram as being two parallel lines that are cut by a transversal. The angles depicted are considered corresponding angles. If two parallel lines are cut by a transversal, then corresponding angles are congruent. We can now set these two angles equal to one another and solve for $x$: $x + 9 = 2x - 21$ Subtract $x$ from each side: $9 = x - 21$ Add $21$ to each side of the equation: $x = 30$