Rick S. answered 11/06/13
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First I need to verify that you are rotating clockwise 2 times by a measure of (5pi)/6.
A clockwise rotation would be subtracted from the original angle.
(-5pi)/6 - (5pi)/6 - (5pi)/6 = (-15pi)/6 = -2((3pi)/6
Each rotation around the axis is 2pi so -2((3pi)/6 = (-3pi)/6 = (-pi)/2