Precalculus: Mathematics for Calculus, 7th Edition

$x=6$
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 $\displaystyle \tan\theta=\frac{y}{x}$. --------- $r\cos\theta=6$ Substitute $r\cos\theta$ with x, $x=6$ ...the equation with rectangular coordinates.