Factorise the following:

Solution:
20 - 45 (m+n)²
20 - 45 ( m² + n² + 2mn ).                [ (a+b)² = a² + b² + 2mn ]
5 [  4 - 9 (m² + n² + 2mn ) ]
5 [ 4 - 9m² - 9n² - 18mn ]                  [ - × + = - ]