Please help find the correct sum. Thank you.

(f + g)(x) = f(x) + g(x)    

Since you know the equations of f(x) and g(x), substitute/plug it into the equation:

f(x) + g(x)     Plug in (5x³ - 7x² - 2x) into "f(x)", and (-3x³ - 9x - 3) into "g(x)"

(5x³ - 7x² - 2x) + (-3x³ - 9x - 3)        Combine like terms(terms that have the same variable and power/exponent)

5x³ - 3x³ - 7x² - 2x - 9x - 3      (I rearranged the terms to be next to their like term, the only like terms you can combine are 5x³ and -3x³, -2x and -9x)

2x³ - 7x² - 11x - 3      Your answer is the 1st option