1. Add the polynomials (9y - 7x + 15a) and (-3y + 8x - 8a) by grouping like terms to get 12y - 15x + 23a. 2. Subtract the polynomials (7a - 10b) and (3a + 4b) by rewriting subtraction as addition of opposites and grouping like terms to get 4a - 14b. 3. Add or subtract polynomials in column form by lining up like terms and adding or subtracting the coefficients.