Factorise :

12(a + b)² - (a + b) - 35

12(a + b)² - (a + b) - 35

= 12(a + b)² - (21 - 20)(a + b) - 35

= 12(a + b)² - 21(a + b) + 20(a + b) - 35

= 3(a + b)(4(a + b) - 7) + 5(4(a + b) - 7)

= 3(a + b)(4a + 4b - 7) + 5(4a + 4b - 7)

= (4a + 4b - 7)(3(a + b) + 5)

= (4a + 4b - 7)(3a + 3b + 5)

Hence,12(a + b)² - (a + b) - 35 = (4a + 4b - 7)(3a + 3b + 5)