Answer
$r=3+3cos\theta$
Work Step by Step
1. Based on the given graph, we can see that the elongation is horizontal and we can select the model $r=a+b\ cos\theta$
2. Use the point $(6,0)$, we have $a+b=6$
3. Use the point $(3,\frac{\pi}{2})$, we have $a=3$, thus $b=3$
4. We have the equation $r=3+3cos\theta$
