SOLUTION: Find a formula for an for the arithmetic sequence. a1 = −1, a5 = 23

Between the first term  a1 = -1 and the fifth term a5 = 23, there is the distance of 24 units and 5-1 = 4 gaps 


of equal length.   Hence, each gap is   = 6 units long.


Thus the common difference of this arithmetic progression is  6 units.


Then the formula for the n-th term of the progression is


     =  + (n-1)*d = -1 + 6*(n-1) = 6n -7.


ANSWER.  The formula for the n-term is   = -1 + 6*(n-1)  or   = 6n -7

         You can chose any of these two versions: they are equivalent.