( c + 6d)^2 - (6c + d)^2 = c^2 + 12cd + 36d^2 - ( 36c^2 + 12cd + d^2) =
= c^2 + 12cd + 36d^2 - 36c^2 - 12cd - d^2 = ( c^2 - d^2) + ( 36d^2 - 36c^2)=
= ( c^2 - d^2) + 36( d^2 - c^2) = ( c^2 - d^2) - 36( c^2 - d^2) = ( c^2 - d^2)(1 - 36) = - 35( c^2 - d^2) = - 35( c - d)(c + d).