In order to sum two polynomials, we can group the powers with the same exponent:
f(x) + g(x) = 8 - 4x - x³ + x² + 7x - 9
= (8 - 9) + (-4x + 7x) - x³ +x²
= -1 + (7 - 4)x - x³ + x²
= -1 +3x - x³ + x²
= -x³ + x² + 3x - 1
Notice that, in the last step, we just wrote the result in the standad form beginning with the power with larger exponent, then the next larger, and so on.