Answer: for 21: a² - b²
Step-by-step explanation:
1. you multiply (a + b) ( a - b)
first you multiply a times a, gives you a²
multiply a times negative b, gives you -ab
multiply b times a, gives you ab
multiply b times b, gives you b²
you have -ab and ab, they will cancel out leaving you with
= a²-b²
