Answer:

y³ + a² y - 9y + 3a²

Step by step solution:

First Step

( y + 3 ) ( y² - 3y + a² )

↓

↓ Multiply the second parentheses by each term

↓ from the first parentheses.

↓

y × ( y² - 3y + a²) + 3 ( y² - 3y + a² )

Second Step

y × ( y² - 3y + a²) + 3 ( y² - 3y + a² )

↓

↓ Distribute y through the parentheses

↓

y³ - 3y² + a² y + 3 ( y² - 3y + a² )

Last Step

y³ - 3y² + a² y + 3y² - 9y + 3a²

↓

↓ Since two opposites add up to zero, removed

↓ them from the expression

↓

y³ + a² y - 9y + 3a²