first, you take the y = -2x and plug that into y = x + 3. so your equation should be -2x = x + 3. you then solve for x which comes out to be -1. you then plug the -1 into y = -2x. your equation should be y = -2(-1). solve for y which equals 2. the point that you got from both equations matches the point from the equation given so it is true.