If you study the following figures,
+ 3 6 [first line]
9 [second line]
and
+ 2 4 [first line]
6 [second line]
and if I show you this figure,
+ 5 3 [first line]
_____ [second line]
what do you think should be written in the second line?
if your answer is 8 then congratulations. You have just learn a little of LISP Programming Language. Your brain discovered the answer though pattern recognition.