you have to find the value of x. first get x by its self on one side of the equation. so you have to add 15 to the -15 to cancel it out. what you do to one side of the equation you have to do to the other side so add 15 to 3x + 12. you should have 4x = 3x + 27. now since you only want the x on one side of the equals sign you have to subtract 3x from both sides of the equation to cancel out the 3x. you should have 1x = 27 or x = 27. now all you have to do is plug in the value of x ,27, into the original equations. 4(27) - 15 = 3(27) + 12.