(4m+7)(7-4m)-(4m-7)^2 = (7 +4m)(7-4m)-(4m-7)^2 = 7^2 - (4m)^2 - 16m^2 - 56m + 49 =  49 - 16m^2 - 56m + 49 = -16m^2 - 56m + 98 = 16m^2 + 56m - 98