book Maths Puzzle Numerical Ability

ok i have done 21 of these and here are the ones i cant get...

Use log9^7=0.8856 and log9^4=0.6309 to evaluate each expression

log9^28
log9^49
log9^144
log9^15.75

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book Other Question

if order does matter, how many ways can you interview the 17 volunteers 3 at a time?

please help me with this question
how do you do this problem?