what is the product of complex numbers (-3i +4) and (3i +4)??? please explain

product of complex numbers (-3i +4) and (3i +4)

(x - y)(x + y) = x² - y²

(-3i +4)(3i +4) = (4 -3i )(4 + 3i) = 4² - (3i)² = 16 - 9*(-1)=16 + 9 = 25