This question is worded a little funny, I agree with the other tutor’s post.
Definition Supplementary Angles: two angles when added together total 180 degrees
There is not a way for me to sketch in this post so I will try to be specific with my wording.
If you have angle REC, then E is the center of this angle with segments ER and EC.
Angle BIG would similarly have a center at I, with segments IB and IG.
If you are to put the segments EC and IB right next to each other you should now be able to see that you will have a straight line (180 degrees) connecting R to G.