2. The square difference formula of exchange position can be (3 (2x-y)+2 (x+2y)) (3 (2x-y)-2 (x+2y)) = (8x+y) (4x-7y).

3. The square difference formula can be changed to (x+2y)(x-2y)=-6.

4. The formula (4m+n)(-2m+3n)=-900 can be obtained as the third question.