IB HL Math Portfolio?

Ok, in part two of the IB HL Math Portfolio, it asks you to define the equation of the shadow generating function for the two cubics y1 and y2. Now, when the two functions were quadratic, the shadow generating function was linear. My question is: Is the shadow generating function of the two cubics also linear? One might think quadratic, but you only have two points to define the function, and I suspect its linear, but I would appreciate some confirmation!