Question

express log9 (xy) in terms of log3 x and log10 y

Answer 1

since 9=3^2,
log9(x) = 1/2 log3(x)
to change bases from 9 to 10 is not so easy:
log9(y) = log10(y)/log10(9)
so you have
1/2 log3(x) + log10(y)/log10(9)
seems like a rather silly exercise