Answer:
x² + 18xi - 81
Step-by-step explanation:
FOIL - First, Outside, Inside, Last
Step 1: Write out expression
(x + 9i)²
Step 2: Expand
(x + 9i)(x + 9i)
Step 3: FOIL
F - x²
O - 9xi
I - 9xi
L - 81i²
Step 4: Combine
x² + 18xi + 81i²
Step 5: Simplify
i² = -1
x² + 18xi + 81(-1)
x² + 18xi - 81
