Step-by-step explanation: They are giving you a value for X that is 3y. You put that value in place of x. So, if x=12 and they want you to substitute for x-3y=0, you would have 12-3y=0....but the value of x in your question is 3y....so you replace the x in the equation with it's value...which is given to you as 3y....or in my example, 12.
