function [x] = SubsDesc(A,b)
  n = size(A,1);
  x(n) = (1/A(n,n))*b(n);
  k = n-1;
  while k>0
      suma = 0;
      for j=k+1:n
          suma = suma + A(k,j)*x(j);
      end
      x(k)=(1/A(k,k))*(b(k)-suma);
      k=k-1;
  end
end