(2a-2b+3c) (2a+2b-3c)
=2a*(2a-2b+3c)+2b* (2a-2b+3c)-3c* (2a-2b+3c)
=(4a方-4ab方+6ac)+(4ab-4b方+6bc)-(6ac-6bc-9c方)
=4a方-4b方+12bc-9c方