Question 612898
mr + ns - nr - ms 


mr - nr + ns - ms 


(mr - nr) + (ns - ms)


r(m - n) + (ns - ms)


r(m - n) - s(-n + m)


r(m - n) - s(m - n)


(r - s)(m - n)


(m - n)(r - s)


So mr + ns - nr - ms completely factors to <font color="red">(m - n)(r - s)</font>