Just need to substitute (with some patience) x=-5 and see whether you get f(x)=0, and x=3.
Now, a parabola is also f(x) proportional to (x+5)*(x-3) (if you know the solutions, which you do in this particular example). So: x^2 + 2*x - 15. This is the third one. The -2 is there to confuse, any number could be there because in this example since f(x)=0 for x=-5 and x=3, it won't change 0*number=0.
