Question

I dont know why people find 21 and 17 38 funny plz explain

Answer 1

Not sure about 21, but the 1738 is a turned used by a rapper named Fetty Wap in his song called Trap Queen to represent the "Remy Boyz 1738" which they named themselves that after the finest 1738 Remy Martin liquor. 
Hope that helps you! :)

Answer 2

21 came from a famous vine, look up on youtube " whats 9+10 vine" and that should give you the answer for 21 :)