(x + 9i)(x − 9i)

To solve the expression "(x + 9i)(x - 9i)", you can use the formula for the difference of squares, which states that a^2 - b^2 = (a - b)(a + b). Here, a is x and b is 9i.

Step 1: Apply the formula (x + 9i)(x - 9i) = x^2 - (9i)^2

Step 2: Simplify the equation = x^2 - 81i^2

Step 3: Since i^2 = -1, substitute -1 for i^2 = x^2 - 81(-1)

Step 4: Simplify further = x^2 + 81

So, (x + 9i)(x - 9i) simplifies to x^2 + 81.