In polar coordinates, the x variable is given as
[tex]x=r\cos \theta[/tex]So, we have the equation
[tex]r\cos \theta=15[/tex]By dividing both sides by cosine of thetat, we get
[tex]r=\frac{15}{\cos \theta}[/tex]since
[tex]\sec \theta=\frac{1}{\cos \theta}[/tex]The above result is equivalent to:
[tex]r=15\sec \theta[/tex]