Explanation:
x and (2x + 9) are supplementary angles.
that means together they have 180°.
why ? because the sum of all angles around a single point on one side of a line is always 180°. because that simulates a half-circle with the center at this point.
and a half-circle has 180°.
so,
x + 2x + 9 = 180
3x = 171
x = 57