Radians

11pi/6 + 3pi/4 = 7pi/12 

Let's get a common denominator and then add the fractions.

So we have

22pi/6 + 9pi/12 = 31pi/12 

But, notice something....that this is just 

24pi/12 + 7pi/12  .....And 24pi/12 is just 2pi.

So, in effect, 31pi/12 is the same "reference" angle as 7pi/12. We've just added another full rotation (2pi) to 7pi/12...

Does that make sense???

In purely numerical terms the left and right sides are not equal; but when interpreted as rotations about a point they give rise to the same net rotation as Chris has explained.  Here is a picture that might help to clarify this.