reserve a,a1,a2,a3,b,b1,b2,b3,r,s,t,u for Real;
reserve n for Nat;
reserve x0,x,x1,x2,x3,y0,y,y1,y2,y3 for Element of REAL n;

theorem Th13:
  (s - t - u)*x = s*x - t*x - u*x
proof
  thus (s - t - u)*x = (s - t)*x - u*x by Th11
    .= s*x - t*x - u*x by Th11;
end;
