If you can't figure out how to do it, you can always use your answer choices, so for example, use choice a (21 degrees). Plug in 21 for x for both equations. So for the first equation that would be 7(21)-1= 146 and second equation 3(2)-9= 54. Then you would add the results, 146+54=200. In that case, choice 21 is wrong because both of these angles should equal to 180 since it is a supplementary angle. I hope that helped.
