This factors out to

.
First, notice that the greatest common factor of this set is

.
Once you factor that out, you have a quadratic

.
You can solve this by factoring using the ac method, to find two factors whose product is -42 and whose sum is 11. These factors are 14 and -3.
You can split the middle term b to write

.
Then, you can factor to write

. Finally, don't forget the 3x from earlier!
This factors out to

.