khan academy is giving me only this explanation but won't go into detail and I cannot understand it for the life of me. how do I know what number to multiply both variables by? what exactly does expanding the parentheses mean? help

12 - 4i

Step-by-step explanation:

-2i ( 2+6i)

Distribute

-2i * 2 + -2i *6i

-4i - 12 i^2

We know that i^2 = -1 so replace it

-4i - 12(-1)

-4i + 12

Then put in the order a+bi  where the real number comes first and the imaginary number comes last

12 - 4i