If the difference is always -4.5, then it is always -4.5 times something, so that it changes -4.5 ever 1 number. This means that our equation is y=-4.5x. From there, we can add any constant, as it will not change the difference. I will leave that up to you to pick, so the equation is y=-4.5x+C, where C is any real number.
Hope this helps!