Evaluate(2+j4)*(-3-j4) and express in polar coordinate fform

Answer 1

I will assume your expression is a complex number of the form a + bi , let me know if you give it a different meaning.

(2+j4)*(-3-j4)
= -6 - 8j - 12j - 16j^2
= -6 - 20j + 16
= 10 - 20j

tanØ = -20/10 = -2, with Ø in quad IV
Ø = 5.176
argument = √(10^2 + (-20)^2) = 10√5

so we could write the result as
10√5cos 5.176 + 10√5sin 5.176
or
in short form: 10√5 cis 5.176

Answer 2

or, we could convert to polar first:
2+j4 = √20 cis 1.107
-3-j4 = 5 cis 4.068
multiply and you get Reiny's value.