Explanation:
5.
angle STQ is the supplementary angle (together they are 180° below line MQ) to 2x + 8.
above ST we have a group of 3 angles that combine to 180° :
2x + 8
90°
71 - x
so, to get x first :
2x + 8 + 90 + 71 - x = 180
x + 169 = 180
x = 11
so, the angle MTS =
2x + 8 = 2×11 + 8 = 22 + 8 = 30°.
6.
therefore, the angle STQ is
180 - angle MTS = 180 - 30 = 150°.