Factorial is a pretty simple concept. it is where you take the starting number (x), and then multiply it by all the numbers below it, until the number below it is 0. So, for example, it would be (x)*(x-1)*(x-2)*(x-3), until 0. this works with negatives as well though. This works with numbers -21 to 21, but after those or before those numbers it will start putting them in a form like this: Xe+Y. The reason it does this, is because the numbers are just too big.
here's how to convert the Xe+Y into a more understandable number: This represents the component exponentiation, where e is the base, and Y is the exponent. In this case, the E just equals 10. Let's use 1.2e+9. This can easily be changed into 1.2*10^9. But if you want to simplify it more, all you have to do is move the '1.2' 9 places to the left. So this would be 1.2 billion, or 1,200,000,000.
