If you have to find the sum of the polynomials \((7x^3 - 4x^2) + (2x^3 - 4x^2)=7x^3-4x^2+2x^3-4x^2\), then you have:
1. group all terms that have equal degrees
\(7x^3+2x^3 \text{ and } -4x^2-4x^2\).
2. Add the coefficients near terms of equal degree
7+2=9,
-4-4=-8.
3. Write the sum as \(9x^3-8x^2\).
Answer: correct choice is D.
