Hi Scy,
7100 = 23600(1-0.0875)t A = P(1 + r)t
7100 = 23600(0.9125)t
(7100/23600) = 0.9125t Divide by 23600
log (7100/23600) = log (0.9125)t Log of both sides
log (7100/23600) = t log (0.9125) Properties of Logs
log (7100/23600)/log (0.9125) = t ≈ 13.1 Divide by log (0.9125)
about 13.1 years
Enjoy