Write the expression as a polynomial: (a-1) (a-4) (a+5)

a³ - 21a + 20

Step-by-step explanation:

expand the first pair of factors using FOIL

= (a² - 5a + 4) (a + 5)

Now, multiply each term in the first factor by each term in the second factor

= a (a² - 5a + 4) + 5 (a² - 5a + 4) ← distribute parenthesis

= a³ - 5a² + 4a + 5a² - 25a + 20 ← collect like terms

= a³ - 21a + 20