Respuesta :

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