How do you do this? ? explain it well !

First you want to combine like terms. on the first equation we can combine 2x+x=3x. So now we have 3x-3. On the right side we can combine 12+1 so we now have -x+13. Now we will take add an x to -x so it's 0 but we have to do it to the other side as well so now we have 4x-3=13. now we will add three to -3 to make it 0 but again we have to do it to the other side. Now we have 4x=16. We want to figure out what x is so we now divide 4x by 4 so we are just left with x but we need to do it to the other side so your answer is x=4