the polar form is r - 2a cos 0 = 0!
to convert a cartesian equation to a polar equation, we use the following,
x = r cos 0 and y sin 0
x2 + y2 - 2ax = 0 ➡️
r2 cos2 0 + r2 sin2 0 - 2ar cos 0 = 0
simplifying..
r2 - 2ar cos 0 = 0
r - 2a cos 0 = 0
hope this helps!!