Explanation:
I am not sure what you want to calculate as ultimate goal.
but your have an error already in your third line in the picture.
(3-6i)(2-4i) = 3×(2-4i) - 6i(2-4i)
you had there a "+" instead of a "-".
and then you made subsequent mistakes in every line, some of them are funnily bringing you more back to the real result (e.g. -12i + 12i would be 0 and not -24i, but if the third line would have been correct, then yes, -24i is actually needed) - but not completely.
= 3×2 - 3×4i - 6i×2 + 6i×4i =
= 6 - 12i - 12i + 24×-1 = 6 - 24i - 24 = -24i - 18
then in the last 3 lines you kind of lose it completely. I am absolutely not sure, what you are calculating, and where the "+" cube from instead of "×" (multiplications) and such.
(3+6i)(2+4i) = 3×2 + 3×4i + 6i×2 + 6i×4i =
= 6 + 12i + 12i + 24×-1 =
= 6 + 24i - 24 = 24i - 18
is that what you wanted to show ?