Answer 1

list comprehension
https://code.sololearn.com/cS2B4H95hCqd/?ref=app

Answer 2

it makes a list, x is just a variable name. So It means "number" for "number" in (the list) 'a', if that number is less than 5

Answer 3

∆BH∆Y

your question is not clear..
You want this ?

https://code.sololearn.com/cHefDOMLCsW6/?ref=app