When you ask "what do you add to A to get B, you solve it by subtracting.
For example, what do you add to 5 to get 13?
Subtract 13 - 5 = 8
The answer is you add 8 to 5 to get 13.
Now you need to do the same with your polynomials.
What do you add to x^2 - 2x + 6 to get 3x^2 + 7x?
To find the answer, subtract
(3x^2 + 7x) - (x^2 - 2x + 6)