function [ x ] = gw_bs2(A)
x=zeros(3,1);
for j=1:3
    y(4-j,1)=A(4-j,4);
    for k= 5-j:3
        y(4-j,1)=y(4-j,1)-A(4-j,k)*x(k,1);
    end
    x(4-j,1)=y(4-j,1)/A(4-j,4-j);
end
end

