Question

(7 - 9i)(12 + 3i)(-5 - 3i)

I know the answer is -816 + 102i I am not understanding how to do the steps

Answer 1

Let's first multiply 7-9i with 12 +3i
\[(7-9i)(12+3i) \\7(12)+ (-9i)(12)+7(3i)+(-9i)(3i)\\84 +21i-108i+27\\111-87i\]
then multiply -5-3i with it
\[(111-87i)(-5-3i)\\111(-5)+111(-3i)+(-87i)(-5)+(-87i)(-3i)\\-555-333i+435i-261\\-816+102i\]

Use the regular foil method \[(a+b)(c+d)=ac+ad+bc+bd\] and just remember that i times i is -1

Answer 2

Thank you soooo much!! That makes more sense now. Do you think you could help me with another problem?

Answer 3

sure go ahead and post it

Answer 4

Oki I posted it