The answer is H because you cannot add together something that is x^{2}, but you can add together -9 and 6 which would make the equation 14x^{2} + 11x - 3. The 11x has + because it is not stated whether it is positive or negative when added.
The student who picked g is wrong because you cannot add together the two x's with coefficients because one has an exponent and the other does not.