Write the equation of an ellipse with one focus at the pole and vertices at the polar coordinates (3, pi) and (9, 2pi).

I found the eccentricity (.66), but don't know where to go from here

r =a(1-e^2)/{1-ecostheta)

theta = pi

r = a(1-e) = 3

theta = 2pi

solve for a and e

(1-e)/(1+e) = 1/3

3-3e = 1+e

e=1/2

a = 6

the equation is

r = 18/4/(1-0.5*costheta)

=18/(4-2costheta)

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