Traditionally, in polar coordinates, the variable " r " is the distance from the origin in a coordinate plane. In polar coordinates, the radius r should be non-negative to describe non-empty set of points. In your case, with r negative r= -3, it describes EMPTY set of points.
"r" CANNOT be negative, so IGNORE the person who claims that it can be.
It can be 0, but not < 0. In other words,