If (m, m) is the solution of the equation: ax + by + (t - s) = 0 ; bx + ay + (s - r)=0 then which

If (m, m) is the solution of the equation: ax + by + (t - s) = 0 ; bx + ay + (s - r)=0 then which of the following must be true?
(i) 2r = s + t (ii) 2s = r + t (iii) 2t = r + s (iv) r + s + t = 0