To simplify the expression [tex]\((x+9)(x-9)\)[/tex], we can use the difference of squares formula. The difference of squares formula states that:
[tex]\[
(a+b)(a-b) = a^2 - b^2
\][/tex]
Here, [tex]\(a = x\)[/tex] and [tex]\(b = 9\)[/tex]. Applying the formula, we get:
[tex]\[
(x+9)(x-9) = x^2 - 9^2
\][/tex]
Now, calculate [tex]\(9^2\)[/tex]:
[tex]\[
9^2 = 81
\][/tex]
So, the expression simplifies to:
[tex]\[
x^2 - 81
\][/tex]
Therefore, the correct answer is:
[tex]\[
\boxed{d. \ x^2-81}
\][/tex]
