log2(9^x-4)=log2(3^x-2)+log2(8)
log2(3^2x-4)=log2(8×3^x-8×2)
3^2x-4=8×3^x-16
设a=3^x
a^2-4=8a-16
a^2-8a+12=0
a1=2,a2=6
∴3^x1=2,3^x2=6
x1=log(3)2
x2=log(3)6