Using the formula for present worth, a=y/(1+n)^t, in how many years will it take for $50,000 accumulate to $70,000 at 15% interest?
I am going to ignore your formula, since the variables are not defined, instead your problem becomes
50000(1.15)^t = 70000
1.15^t = 1.4
log (1.15^t) = log 1.4
t log 1.15 = log 1.4
t = log 1.4/log 1.15 = 2.4 = 2 years and about 5 months