If [(3^n)-1]/2 = 900

show that n = [log(900) / log(3)] +1

This is from a past A/S Level C2 paper and I can't get it to come out the way they want.

The closest I can get is that n= [log((900 x 2) + 1)] / log(3)


Can anyone show me how to put in in the way they asked for?