The equation for constant of proportionality is y=KX. the K stands for the constant of proportionality and since the constant proportionality is 1.5 you will take 1.5 * x and for example x is 2 so you do 1.5 * 2 and you get 3 and 3 is your y. If you plug in the equation you're saying 3 equals 1.5 * 2 which is right.
