Add the polynomials.(7x^2-6x+6)+(3x^3-9x)

A polynomial in a variable x is a function of the form ${\displaystyle a_n{x}^n+a_{n-1}{x}^{n-1}+\dots +a_{1}x+a_0}$. This polynomial has degree n. The constants ${\displaystyle a_i}$ are the coefficients and are constants.
To add two polynomials we add like terms, that is, add terms of the same degree. For example ${\displaystyle 2x^2,4x^2}$ are terms of the same degree but the terms ${\displaystyle 2x^2,4x^3}$ are terms of different degree.
The sum to be computed is ${\displaystyle (7x^2-6x+6)+(3x^3-9x)}$. . Drop the parenthesis and combine like terms.
${\displaystyle (7x^2-6x+6)+(3x^3-9x)=7x^2-6x+3x^3-9x}$
${\displaystyle =3x^3+7x^2+(-6x-9x)+6}$
${\displaystyle =3x^3+7x^2-15x+6}$
Hence, the sum is ${\displaystyle 3x^3+7x^2-15x+6}$.