Book chapter 3 exercise 3.1 is move to ch3_2
Draft version of homework week 3
make hist graph smaller, fix data error in 3.1, make data.frame print pretty.
Add normal line in 3.2
Finish exercise 3.2
Finish exercise 3.4
Finish exercise 3.9
Finish exercise 6.1
Change work to fit RStudio in windows