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.

If i could just have the statements, i'd be able to