we can plug your numbers in and get 3 equations in 3 unknowns, a, b, and c, which can be solved for.
if you do this you find that the stock price is modeled by an inverted parabola, i.e. a<0, that intersects the x axis at x=125.498.
I guess you would just say that the stock price is 0 after 125.5 days
Give a yell if you can't figure out the coefficients.