Since (x - 9)° and (5x + 3)° represent a pair of supplementary angles, then their sum must be equal to 180°.
Therefore, (x - 9) + (5x + 3) = 180
x - 9 + 5x + 3 = 180
x + 5x - 9 + 3 = 180
6x - 6 = 180
6x = 180 +6
6x = 186
x = 186/6
x = 31°