Brainliest, 10 points

Answer:
last option is correct
Step-by-step explanation:
(a-9)(a+9)
a(a+9)-9(a+9)
a^2+9a-9a-81 (plus 9a and minus -9a gets cancel)
a^2-81
a^2-9^2
Answer:
[tex]a^2-81[/tex]
Step-by-step explanation:
[tex](a-9)(a+9)[/tex]
Distribute, multiply everything in one of the parenthesis by everything in the other parenthesis,
[tex](a - 9)(a + 9)\\\\(a)(a)+(-9)(a)+(9)(a)+(-9)(9)[/tex]
Simplify, perform the multiplication and combine like terms,
[tex]a^2-9a+9a-81\\\\a^2-81[/tex]