Answer
$x=6$
Work Step by Step
Formulas relating rectangular $(x,y)$ and polar $(r,\theta)$ coordinates:
$ x=r\cos\theta$ and $ y=r\sin\theta$
$r^{2}=x^{2}+y^{2}$ and $\displaystyle \tan\theta=\frac{y}{x}$.
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$r\cos\theta=6$
Substitute $ r\cos\theta$ with x,
$x=6$
...the equation with rectangular coordinates.