2.3=(1.05)^n
2.3=(1.05)^n take the log of each side
log 2.3 = log 1.05^n and by a log property, we can write
log 2.3 = (n) log 1.05 divide both sides by log 1.05
log 2.3 / log 1.05 = n = about 17.07