You can see that you're constantly subtracting 8 to get the next number. So, we have
[tex]a_1 = 15[/tex]
[tex]a_2 = a_1-8[/tex]
[tex]a_3 = a_2-8 = (a_1-8)-8=a_1-2\cdot 8[/tex]
[tex]a_4=a_1-3\cdot 8[/tex]
[tex]a_5=a_1-4\cdot 8[/tex]
So, as you can see, to get the n-th term we must subtract (n-1) times 8 from the original number.
