You asked about the "best solution". The best solution comes from an exponential-growth model:
Price = 16.5 * 2^(t/35)
where t is the number of years that have elapsed since January 1, 1996.
I do not know what your son's class is doing, but, if the point of this exercise is to estimate a solution, then your linear-growth approach may be acceptable.
Your result is only two years less than the "best solution". :cool: