By observing the points you can learn a lot about a function. Concretely [tex]f(x)[/tex] passes through [tex](1,1)[/tex] but [tex]g(x)[/tex] passes through [tex](1,-\frac{1}{2})[/tex] that should give you a hint that [tex]g(x)=-\frac{1}{2}x^2[/tex].
Hope this helps :)
