Answer
$$(5x-2i)(-4x)^3=128ix^3-320x^4$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}(5x-2i)(-4x)^3& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}(5x-2i)\cdot-64x^3 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}-320x^4+128ix^3 \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}128ix^3-320x^4\end{aligned} $$ | |
| ① | $$ \left( -4x \right)^3 = (-4)^3x^3 = -64x^3 $$ |
| ② | $$ \left( \color{blue}{5x-2i}\right) \cdot -64x^3 = -320x^4+128ix^3 $$ |
| ③ | Combine like terms: $$ 128ix^3-320x^4 = 128ix^3-320x^4 $$ |
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