Let's start by just simplifying the equation.
3x+5+2x=12+4x becomes 5x+5=12+4x
Now, we want all our X's on one side of the equation. We accomplish this by substracting the smaller x value from the larger x value.
5x-4x=x
x+5=12
Finally, we make sure the x is by itself
12-5= 7
x=7