I'm pretty stuck here.. any assistance would be helpful

given $\displaystyle r=3cos\Theta$, convert to rectangular form

I know that $\displaystyle r^2 = x^2 + y^2$, so I could square both sides

$\displaystyle r^2 = (3cos\Theta)^2$

$\displaystyle x^2 +y^2 = (3cos\Theta)^2$

But at that point I'm stuck. I have the answer from the back of the book, but can't work out how to complete the conversion.