What polynomial must be added to x^2-2x+6 so that the sun is 3x^2+7x?

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)