[n,m] = size(A);
x = zeros(n,1);
x(n) = b(n)/A(n,n);

for k = (n-1):-1:1
    S = 0;
    for j = k+1:n
        S = S + A(k,j) * x(j);
    end
    x(k) = (b(k) - S)/A(k,k);
end

end