Given, sum of 13mn, −7nm, 4m and 7mn is x
⟹ 13mn+(−7nm)+4m+7mn=x [1 Mark]
⟹ 13mn +7mn - 7nm +4m=x
i.e. x = 13mn +4m [1 Mark]
If the sum of 13mn, −7nm, 4m and 7mn is x, then x = 14mn+4m.