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

Answer:
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