Answer
$x=\displaystyle \frac{1}{2}\arctan\frac{y}{3}$
Work Step by Step
The goal is to isolate x.
Divide with 3...
$\displaystyle \frac{y}{3}=\tan 2x\qquad$ ...use the definition of arctan
$2x=\displaystyle \arctan\frac{y}{3}$
(2x is the number from $(-\displaystyle \frac{\pi}{2}, \displaystyle \frac{\pi}{2})$ such that $\displaystyle \tan\frac{y}{3}=2x$)
(...If $2x\displaystyle \in(-\frac{\pi}{2}, \displaystyle \frac{\pi}{2})$, then $x\displaystyle \in(-\frac{\pi}{4}, \displaystyle \frac{\pi}{4})$...)
... divide with 2 ...
$x=\displaystyle \frac{1}{2}\arctan\frac{y}{3}$
