Multiply. (a-9)(a+9) a a^2-18 b a^2-18a-81 C a^2+18a-81 d a^2-81

To multiply the given expression \((a-9)(a+9)\), we can use the difference of squares formula, which states that \((x-y)(x+y) = x^2 - y^2\).<br /><br />In this case, \(x = a\) and \(y = 9\). Applying the formula, we get:<br /><br />\[<br />(a-9)(a+9) = a^2 - 9^2 = a^2 - 81<br />\]<br /><br />So, the correct answer is:<br /><br />d) \(a^2 - 81\)