His height. To explain, if he was a newborn his age, x, would be 0. That can be modeled by y=4.3(0)+19 and so y would equal 19. He was 19 inches as a newborn. When he turned 1, x changed to 1. This can be shown by y=4.3(1)+19 where his height, y, is now 23.3 inches. The 19 stays constant because he was 19 inches as a newborn and every year that he grows older he grows 4.3 inches. Hope this helps!
