3/4 = 9/12, so we can write:
log9(3/4) = log9(9/12)
Now use the log property that log(a/b) = log(a) - log(b):
log9(3/4) = log9(9/12) = log9(9) - log9(12)
Now use the identity log9(9) = 1 and the given value of log9(12) = Q:
log9(3/4) = log9(9/12) = log9(9) - log9(12) = 1 - Q