The first thing you should do is get x on one side.
12+5x=7x+3
12+5x-5x=7x-5x+3
12=2x+3
Now that you've done that you must get the 2x by itself. You can do this by subtracting 3 from both sides.
12-3=2x+3-3
9=2x
Remember, you would only subtract if the number next to x is positive. If it is negative you would add. (Ex. 12=5x-3,12+3=5x-3+3, 15/5=5x/5, 3=x)
Now, you must divide both sides by 2 to get x by itself.
9/2=2x/2
Therefore your answer is 4.5=x
Now you have your X by itself. Hope this helps.