Some identity could be useful for this.
The answer is
I'd know how to do it if it weren't for the 3 infront of arctan(2).
EDIT: I guess I could do it like this:
That seems a bit over the top though. I'm still looking for an easier solution.
Also, I've never encountered three arctan terms before:
I'm guessing I need to first simplify , and then use that to get by using the rule again. And of course make sure it's in the range of