theorem
  M1 is line_circulant & M2 is line_circulant & M3 is line_circulant
  implies a*M1-b*M2+c*M3 is line_circulant
proof
  assume M1 is line_circulant & M2 is line_circulant & M3 is line_circulant;
  then c*M3 is line_circulant & a*M1 - b*M2 is line_circulant by Th6,Th13;
  hence thesis by Th7;
end;
