FV of growing annuity = P * ((1+r)^n - (1+g)^n) / (r-g)
P=initial payment
r=discount rate or interest rate
g=growth rate
n=number of periods
^=raised to the power of
NB: This formula breaks when r=g due to division by 0. When r=g,
use
P * n * (1+r)^(n-1)