Factorise (x+y) ^2 - (4-x) ^2

by using the identity a square - b square = (a+b) * (a-b)

a = x+y

b=4-x

= (x+y + 4-x) (x+y - 4-x)

y+4 * x+y - 4+x

y+4*2x+y-4

2xy+y square - 4y + yx + 4x + 4y - 16 + 4x

3xy + y square + 8x - 16