Find an explicit formula for the arithmetic sequence - 5,13,31,49,

The common difference dividing each term from the next is 18.

Then a (n) = - 5 + (n-1) (18)

Check: find a (1) using this formula. Is a (1) = - 5? a (1) = - 5 + (1-1) (18) does equal - 5.

Find a (2); a (2) = - 5 + (2-1) (18) = - 5 + 18 = 13. Correct.