One thing the video brought up is that even though it looks like the answer is 3,864 you have to consider the numbers after the decimal point that hold value. In the equation the digits .1 and .4 are after the decimal point so you move the decimal point two places to the left. You will get 38.64 instead of 3,864.
I'm sorry about the fact that my keyboard doesnt have a divide button on it.
Remember: Math Links page 57-59 numbers 3-21 isn't homework
Here's an excellent link to a website that explains the concept of multiplying decimals well.