In R2, a vector a≠0 in the second quadrant makes an angle of 120∘ with the positive x-axis, and a vector b≠0 in the third quadrant makes an angle of 150∘ with the positive x-axis (in the sense that the smaller of the two angles between b and the positive x-axis is 150∘). Find the sum of the angle between a−2b and a and the angle between a−2b and −2b.