To find the answer you need to get x by itself. First subtract 3x from both sides, you end up with 4=6x+4. Now you need to subtract 4 from both sides, and you get 0=6x. Then you divide both sides by 6 and you get 0=x.
3x+4=9x+4
-3x -3x
4=6x+4
-4 -4
0=6x
÷6 ÷6
0=x