I am confusing about can i use "Identities" in Complex number term!

For example, i have a Complex Conjugate in multiplication : (2 - 4i)(2 + 4i)

And the answer is 20 if i multiply these two parentheses together. But if i use "identity" (a - b)(a + b) = (a - b)^2 , the answers in two different ways were not same!