Supposing CD uses the equation y=mb (meaning CD passes through the origin) and passes through (3,5), we then know the slope if 5/3 and the y intercept is 0. We can then write CD's equation as y=5/3x+0, or just y=5/3x.
So if AB is parallel to CB, then their equation is y=5/3x+b, as we don't the y-intercept yet.
