In the post, the problem is presented INCORRECTLY, since " tan6 " is 
non-sensical hieroglyph in Math.  In other words, its meaning is undefined.


To give a sense,  " arctan(tan6) "  should be re-written as  " arctan(tan(6)) ".


Then  " tan(6) "  means tangent function of the angle of 6 radians.



The hieroglyph  " arctan(tan(6)) "  then means 

" the angle in the interval  (,),  whose tangent function is the same as tan(6) ".


This sough angle is then the angle of 6 radians, reduced to the interval  (,).


In other words, the  ANSWER  is    radians, or, approximately, 6 - 2*3.14 = 6 - 6.28 = -0.28 radians.