Multiply the equation by r to get
... r² = -14·r·cos(θ)
Then make the usual substitutions:
x² +y² = r²
x = r·cos(θ)
This gives
... x² +y² = -14x
... x² +14x +49 +y² = 49 . . . . . add 49+14x
... (x +7)² + y² = 49 . . . . . . . . . equation of circle of radius 7 with center (-7, 0)
