He always compared the engine to the sound of a cat purring. The word definition for purring is: a low tonal, continual, fluttery sound made by some animal to show or to express a level of contentment or pleasure. For example, cats purr a lot when they are in a state of peace or when they are happy. I guess that is the reason why the motorcycle sound reminded Chan's dad of a cat.
Well the closest one is probably the y intercept=-5 and the slope=3 one but the equations show that the slope is -3 and the y intercept is 5 for the first one.
