Answer: C. (x + 9)(x + 9)
Explanation:
To factor the given expression, we need to find two terms that will equal the expression when added together.
To do this with a quadradic equation that doesn't have a coefficient, we will find two numbers that add to b and multiply to c (ax² + bx + c)
9 and 9 will add to 18 and multiply to 81. These are our factors.
To write this as factored form, we write:
(x + 9)(x + 9)
This works because when you multiply it out, you get an x², two 9xs (add together to 18x), and an 81.