Ah damn, I missed that! Good catch there..Doesn't this method just ignore the -23?
You can substitute known y and x values and rearrange to get the two equations:
27=a*3^n and 243=a*9^n ( simplifies to 243=a*3^2n as 9=3^2)
Divide second equation by first to get 9=3^n and so n=2.
Substitute n back into 27=a*3^2 to find that a=3.