Geometric equations with variable terms

Hello,

I'm not sure if my title is correct terminology but I am stuck. I can work out a geometric progression easily and use the equations. Unfortunately when the equation includes a term that relies on the value of another, I am confused.

What do I do in said situation? Say there are three terms, a + b + c, the value of b is a to a power and c is an exact value.

What sort of steps would I take to work backwards to get the value? (Sleepy)

$\displaystyle GPn = a * ratio^n-1 $

Thanks,

Unmath Sam