So carbon number one is attached to two groups. It's either attached to the H or it's attached to the carbon here. So which atom has a higher atomic number? Well, clearly the carbon has a higher atomic number, and that means this group attached this carbon has a higher priority than this group here. Likewise, let's examine the second carbon. The second carbon of the double bond is also attached to a carbon, and it's also attached to an H.
Which one of these two groups has a higher atomic number?