Regression Question Past Paper

#1
Hey guys im currently doing a past paper for a statistics course and I came across this question I have added images with the answer I understand everything up until part f where I am having difficulties understanding how they attained the answer. Any help will be appreciated.

Exam question: Screenshot 2019-04-08 at 21.21.40.png

answer to part b: Screenshot 2019-04-08 at 21.22.00.png

answer to part c, d and e: Screenshot 2019-04-08 at 21.22.10.png

answer to part f: Screenshot 2019-04-08 at 21.22.16.png
 

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