#### Answer

Group 2: RO
Group 3: R$_2$O$_3$
Group 4: RO$_2$

#### Work Step by Step

Group 2: RO
-The oxidation state for group 2 elements is 2+. If oxygen, with an oxidation number of 2-, is chemically combined with a group 2 element, then we will have a 1:1 relationship between the two elements. Therefore, we can write the general formula for a group 2 element combined with oxygen as RO, with the R representing the group 2 element.
Group 3: R$_2$O$_3$
-The oxidation state for group 3 elements is 3+. If oxygen, with an oxidation number of 2-, is chemically combined with a group 3 element, then we will have a 2:3 relationship between oxygen and the group 3 element, respectively. This means that in a compound containing the two elements, there will be two of the group 3 element's atoms for every three oxygen atoms. Therefore, we can write the general formula for a group 3 element combined with oxygen as R$_2$O$_3$, with the R representing the group 3 element.
Group 4: RO$_2$
-The oxidation state for group 4 elements is 4+. If oxygen, with an oxidation number of 2-, is chemically combined with a group 4 element, then we will have a 2:4 relationship (reduced to a 1:2 relationship) between oxygen and the group 4 element. This means that in a compound containing the two elements, there will be one of the group 4 element's atoms for every two oxygen atoms. Therefore, we can write the general formula for a group 4 element combined with oxygen as RO$_2$, with the R representing the group 4 element.