(a+b)^2-11(a+b)-42

(a+b)² - 11( a+b ) - 42
(a+b)² - 14(a+b) + 3(a+b ) -42
[Splitting Middle Term Method]
(a+b){a+b-14}  +3{(a+b-14)} = (a+b-14) (a+b+3)  [Answer]