(n+m)^2-(n-m)^2
(k-7)^2-(k+7)^2
(m+5n)^2-(5n-m)^2

(n+m)²-(n-m)²=n²+2nm+m²-(n²-2nm+m²)=n²+2nm+m²-n²+2nm-m²=4nm(k-7)²-(k+7)²=k²-14k+49-k²-14k-49=-28k(m+5n)²-(5n-m)²=m²+10mn+25n²-25n²+10mn-m²=20mn