What formula can be used to describe the sequence below?

-8,-5,-2,1,4 ...

This is just an arithmetic sequence because each term is a constant difference from the preceding term, called the common difference.

Any arithmetic sequence can be expressed as:

a (n) = a+d (n-1) where a=initial term, d=common difference, n=term number

In this case a=-8 and d=3 so

a (n) = - 8+3 (n-1) which you can simplify if you'd like

a (n) = - 8+3n-3

a (n) = 3n-11