Answer
$x = 30$
Work Step by Step
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:
$x = 30$
