Factorise :

(x + y)(a + b) + (x - y)(a + b)

(x + y)(a + b) + (x - y)(a + b)

= (a + b)$\Big($(x + y) + (x - y)$\Big)$

= (a + b)$\Big($x + y + x - y$\Big)$

= (a + b)(2x)

= 2x(a + b)

Hence, (x + y)(a + b) + (x - y)(a + b) = 2x(a + b)