I printed the original post. I printed the table. I printed an explicit definition for the symbol x, in sentence form.
I took these three sheets of paper to the college, and I pestered about three dozen people in and about the Math Building's lounge and in the math lab, itself.
Not very scientific, yet after confirming that the respondent had intermediate-algebra experience I asked, "What do you think the symbol x represents, in this chart?" and recorded their response as either "correct or close enough" or "they haven't got a clue".
Subjects privvy to the printed definition answered correctly (or close enough) 94% of the time (16 out of 17).
Subjects shown only the exercise and chart answered correctly (or close enough) 69% of the time (11 out of 16).