First, parallel means a line with the same slope. Because of that there are many options usually narrowed down in your book, make sure it is not perpendicular because that only has one correct answer for each equation given.
for a parallel you use the same slope so you have y=1/2x
if not given any other information like a point it goes through you can put whatever you want to on the end as long as it does not match the original. for example y=1/2x is a correct answer or y=1/2x-2 is also correct