How do you do 18+2x=-8x+6 (7+3x)

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