In general ,you cannot combine logs with different bases.
Log15 base 9 - log 75 base 9 = log_9(1/5) = -log_9(5)
so you have
log_10(450) - log_9(5)
so add 'em up. If you want to use just logs base 10, then that would be
log450 - log5/log9
but I don't see how that's any better
Log15 base 9 - log 75 base 9 + log 450 base 10
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