Since y=x-4 you would simply just plug that into 'y' on the other equation, given: x-4=3x+2. Subtract x from both sides, it will leave you with -4=2x+2. Next you would subtract 2 from both sides, leaving you with -6=2x. All you need now is to get rid of the coefficient in front of 'x' so you divide both sides with 2, leaving you with your answer: -3=x.