Simple polar to cartesian conversion

[Trousers]

Find a Cartesian equation for the curve.
$$18=rcos \theta$$

$$\frac{\ 18}{cos \theta}= \sqrt{x^2+y^2}$$

how do I get rid of the cosine?

 

[HallsofIvy]

Don't divide by it! What is $r cos(\theta)$ in terms of x and y coordinates? In other words, x= ? , y= ? in polar coordinates.

 

[Trousers]

x=18 is the solution. I can't believe it.