Math Help - complete the square

1. complete the square

find the polar equation of the curve x^2+y^2=2cx by first completing the square.
How do I complete the square?
I have not done it in this form before?
Thanks

2. $
\begin{gathered}
x^2 - 2cx + \underbrace {c^2 - c^2 }_{} + y^2 = 0 \hfill \\
\left( {x - c} \right)^2 + y^2 = c^2 \hfill \\
\end{gathered}
$