You start by figuring it will be in the form of (3x + a)(x + b) because 3 doesn't factor. The two middle terms in the line followed by the stars must add up to 14x. And in (3x + a)(x + b), ab must be equal to the 8 on the end (the constant factor).
For most people, factoring takes a LOT of practice. And somethimes you just have to try a lot of factors before you hit the right combination. But if you do it enough, it really does get easier to 'see' the answer a lot of the time.
