Refresh my memory how do you write x=5 in rectangular form?

The first thing you should know in this case is that the equation is written in a rectangular form.
 It can be written in polar form.
 To do this, you must make the following change of variables:
 x = r * cos (theta)
 We have then:
 r * cos (theta) = 5
 r = 5 / cos (theta)
 r = 5 * sec (theta)
 
 The polar form is:
 r = 5 * sec (theta)
 Note: the equation was already written in rectangular form.