是8的X-Y-2Z次方吧?
2^x=3;4^y=2^2y=(2^y)*(2^y)=5得2^y=5/2;8^z=7可知
8^(x-y-2z)=8^x+1/8^y+1/8^(2z)
=2^(3x)+1/2^(3y)+1/(8^z+8^z)
=2^x+2^x+2^x+1/(2^y+2^y+2^y)+1/(8^z+8^z)
=3+3+3+2/(5+5+5）+1/(7+7)
=9+2/15+1/14