assuming you mean remember in form a is leading coficient (h,k) is vertex k will be the minimum or max value of the function if a is positive, then the parabola opens up and k is minimum if a is negative, then the parabola opens down and k is max
given a is negative the parabola opens down and k is the max value it is -3 the max value is -3, which occors at x=2 g(2)=-3 is max