You don't!
No, I don't think your friends will hate you if you make out under 13 but at the same time why would you need to make out under the age if 13?
The two numbers that multiply to make 40 and add to make -13 are -8 and -5. When you multiply -8 and -5, you get 40, and when you add them together, you get -13.
13
13 is the unlucky
You don't. 13 is already prime.
1 x 13 and 13 x 1
Most children under 13 have an attempt to make one faking there age, My opinion is you to wait 'til you are 13 and check with your parents that you can make a Facebook.
Subtract 1999 from it.
338
13
No.
5 will make 12 and 13 a Pythagorean triad.