What is the error of the problem? I don't quite understand but I'll like if anyone can explain it to me :)))

For the following sequence determine the common difference (if it is an arithmetic sequence) or the common ratio (if it is a geometric sequence).

While scuba diving, rajeev dove to a depth of 12.6 feet below the surface of the water and then descended another 8.7 feet. what expression can be used to find rajeev's new position? 12.6 – 8.7 –12.6 – 8.7 –12.6 – (–8.7) 12.6 – (–8.7)

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?