3^a=5,(1)
3^(a-3b)=5/27,(2)
(1)÷(2)
3^a ÷ 3^(a-3b)=5÷（5/27)=27=3³
3^[a-(a-3b)]=3³
3^（3b）=3³
3b=3
b=1
2009^（b-2）=2009^（1-2）=2009^（-1）=1/2009