function [sol info res err]= subsparatras(a,b,exa)
  [m n] = size(a);
  
  if (m != n)
    infor = 0;
    return;
  endif

  if (size(b,1)!= m)
    infor = 0;
    return;
  endif
  
  x = zeros(n,1);
  x(m) = b(m)/a(m,m);
  
  for k = m-1:-1:1
    x(k) = (b(k) - a(k,k+1:m)*x(k+1:m))/a(k,k);
  endfor
  
  res=norm(a*x-b)
  infor = 1;
endfunction