2. extract (x+2)(X-2)y, that is, (x 2-4) y.

3. (m 2+4) (m-2) 2 (m+2) is first converted into m 4 (m-2)-16 (m-2), and then converted into (m 4-16) (m-2).

4. The simplification of (x+4y) (x-4y) can be simplified as x 2- 16y 2.