To be able to factor x + 4 from the last two terms, you need the unit coefficient to be 4 x 18 = 72, so you need to add and subtract 68:
(x + 4)x + 18x + 4 + 68 - 68
(x + 4)x + 18x + 72 - 68
(x + 4)x + (x + 4)18 - 68
So you end up with:
(x + 4)(x + 18) - 68
Answer:
Add and subtract 68
