R=2cosθ Convert to rectangular form?

[Jurrasic]

It does not explain it in any of the books ? So no clue even on how to start other than,
R = X squared + Y squared

 

[eumyang]

It does not explain it in any of the books ? So no clue even on how to start other than,
R = X squared + Y squared

First of all this isn't correct. It should be
r² = x² + y².

Second, if you have a textbook, check it again, because there are at least two other equations that can be used to convert polar equations:
x = r cos θ and y = r sin θ.

As for your problem,
r = 2 cos θ,
start by multiplying both sides by r. Then substitute.