Представте в виде произведения (с+m)2-(c-m)2
(c+m)^2-(c-m)(c+m)=(c+m)(c+m-c+m)=(c+m)2m
(c+m)^2-(c-m)^2=(c+m-c+m)(c+m+c-m)=2m*2c=4mc