Let [tex]a_{1}[/tex] = -9, [tex]a_{2}[/tex] = -12 and so on.
-15 = -9 + (-6)
-15 = -9 + 2(-3)
Let -3 be the common difference, or d.
[tex]a_{n}[/tex] = -9 + n(-3)
= -9 - 3n
