I think it is the Associative Property of Addition.
No an obtuse angle does not have a complement
<em><u>this </u></em><em><u>is </u></em><em><u>the </u></em><em><u>correct</u></em><em><u> answer</u></em><em><u> </u></em>
<span>arctan(2x) = -1 radian
</span>⇒ 2x = tan(-1)
⇒ 2x = -1.5574
⇒ x = -1.5574/2
⇒ x = - 0.7787<span>
</span>