I accidentally just deleted some working and cannot get it back.

Can you help me with this please.

Factor fully: ( a² - b² - c² )² - 4b²c²

= (a²-b²-c²)² - (2bc)²

= (a² - b² - c² - 2bc)(a² - b² - c² + 2bc)

= (a² - (b + c)²)(a² - (b – c)²)

=

=

= (a-b-c)(a+b+c)(a-b+c)(a+b-c)