(a+3b)(a²-3ab+9b²)-(a-3b)(a²+3ab+9b²)
=a³-3a²b+9ab²+3a²b-9ab²+27b³-(a³+3a²b+9ab²-3a²b-9ab²-27b³)
=a³-3a²b+9ab²+3a²b-9ab²+27b³-a³-3a²b-9ab²+3a²b+9ab²+27b³
=54b³