Chapter 10 - Conics, Parametric Equations, and Polar Coordinates - Review Exercises - Page 743: 70

$$r = 6\sec \theta $$

$$\eqalign{ & x = 6 \cr & {\text{Convert to polar form, using }}x = r\cos \theta \cr & r\cos \theta = 6 \cr & \frac{{r\cos \theta }}{{\cos \theta }} = \frac{6}{{\cos \theta }} \cr & r = 6\sec \theta \cr & {\text{The graph represents a vertical line at }}x = 6 \cr & {\text{Graph}} \cr} $$