Add and Subtract the following polynomials:
1. (-7x^2 + 5xy - 2y^2) + (3x^2 - 5xy - 4y^2)
2. (6r^3 - 8rs + 4s^2) - (-5r^3 + 3rs + s^2)
3. (7a^4 + 8ab + b^2) + (-5a^4 - 8ab - 4