(4^m * 8^(m-1)) / 2^m
=(2^2m * 2^(3m-3))/ 2^m
=2^(2m+3m-3)/2^m
=2^(5m-3-m)
=2^(4m-3)
因为2^9=512
所以4m-3=9
所以m=3