2^(x-1)+2^(x-2)+2^(x-3)
=2^2*2^(x-3）+2*2^(x-1)+2^(x-3)
=2^(x-3)*(4+2+1)=448
2^(x-3)=448/7=64
2^6=64
x-3=6
x=9