Determine if the sequence could be arithmetic. if so give the common difference -7, 1,7,13,18,

An arithmetic sequence will increase by  the same amount  called the common difference (d)

Here d for first 2 terms = 1 - -7  = 8 , 
then 7-1 = 6 , then 13/7 = 6 and 18-13 = 5  
so it is not arithmetic.

The year before the 10s so it was like when caveman were alive