If this is a simultaneous equation then:
you should simplify it to get 18+2x=-8x+42+18x
Then simplify again and you get: 18+2x=10x+42
Next if you minus 2x from both sides you get: 18=8x+42
After that you minus 42 from both sides to get: -24=8x
Finally, divide both sides by 8 and you get: -3=x