Two vectors 2a+b and a=3b are perpendicular. Find the angle between a and b, if |a| = 2|b|.

Answer 1

Is a = 3b supposed to be a - 3b? or a + 3b?

If a = 3b is one of the vectors, then you would be saying that a and b are collinear, with |a| = 3|b|. That would contradict your later statement that |a| = 2|b|