How do you graph ##r=-3##?

Allowing that r could be negative (against the befitting definition that, as length (modulus ) of a vector, ##r>=0) , r =-3## represents the circle with center at the pole r = 0 and radius = 3..

Despite that r is understood as length (modulus ) of the position

vector from pole to the position ##(r, theta)##, it is customary to allow r

to be negative, by identifying the point ##(+r, theta)## as

##( -r, theta+pi)##.

So, both ##r=3## here and r=-3 represent the same circle with center

at the pole r = 0 and radius = 3.

The point ##(3, theta)## on the circle from the first equation is (-3,

theta+pi), from the second. .