1. 12a²b² + -7a²b² + 9a²b²
= 14a²b² (the terms a²b² are like terms so don't look at them, just add 12 + (-7) + 9.
= 14 (ab)² (you can write like this too because both a & b are squared).
2. y² - (-5y²)
= 6y² (subtract -5 from 1, y² is the like term).
