(3x^2 + 2x + 7X^3) + (10x^2 + 6x + 9)
Combine all the like terms.
There is only one term with X^3 so that stays the same.
Add 3X^2 and 10x^2 to get 13x^2.
Add 2x and 6x to get 8x.
There is only one term with a variable, so that also stays the same.
Now place them in order from highest exponent to lowest:
7x^3 + 13x^2 + 8x + 9
The answer is A.