To add polynomials, combine the like terms, then follow the rules on adding integers.
Like terms are terms with same literal coefficients.
Literal coefficient is variable or variables and its exponent.
Example:
4x³ - 4xy² + xy² - 2x³
4x³ - 2x³ - 4xy² + xy²
Combine 4x³ and -2x³ = 2x³
Combine -4xy² and xy² = -3xy²
The result is:
2x³ - 3xy²