If (x-1) is a factor of then it will divide into it evenly. So we will use Synthetic Division to solve this:
1 | 3 1 2k 4
--- 3 4 2k+4
------------------
3 4 2k+4 2k+8
The remainder, we can see above, is 2k+8. Since we know that (x-1) divides evenly, the remainder must be zero:
2k+8 = 0
Now we solve this for k. Subtract 8 from both sides:
2k = -8
Divide both sides by 2:
k = -4