What separates terms in an algebraic expression

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie

Which equation results from adding the equations in this system? x+6y=9 and -x+2y=-15

For a science project, a high school research team conducted a survey of local air temperatures. based on the results of the survey, the found that the average temperatures were around 10 degrees higher than expected. this result was wrong. the trouble with the survey was that most of the locations were exposed to direct sunlight and located over asphalt or sand, which resulted in higher temperatures than normal. this is a classic example of an error in which phase of inferential statistics?