Hello, paulac!
If you are solving for \(\displaystyle x\):
\(\displaystyle \L \;\,-\,x\,+\,12\,=\,-\,4\,+\,x\)
Add \(\displaystyle x\) to both sides:\(\displaystyle \L \;12\,=\,-\,4\.+\,2x\)
Add \(\displaystyle 4\) to both sides:\(\displaystyle \L \;16\,=\,2x\)
Now finish it up!