(2a+3b-c)(2a-3b+c)
={2a+(3b-c)}{2a-(3b-c)}
=4a^2-(3b-c)^2
=4a^2-9b^2+6bc-c^2
(a+2)的平方-2（a+3）（a-2）+（a-3）的平方
={a+2-(a-3)}^2
=5^2
=25