How do you do this? ? explain it well !

First what you need to do is distribute the properties. Take 2x times the -3 and you will get -1x. Then take the 12 and times it time x and you will get -12x, and then times 12 times 1 which is 12, now here's a fact so you don't get confused, instead of the plus and minus signs it's a negative and positive signs) then you take -1x and -12x and divide them by each other and you will get 12, X=12