At 13:26 in this youtube video why/how did he:
1) Change the three negative fractions to positive ones
2) Change the denominator from 4 to 8 on the last fraction
I think it is because he did a half multiplied by everything inside the bracket which turned them from negative into positive, and changed the denominators?
He had $y=\frac32-\frac12(-\frac14-\frac{3\sqrt{17}}4)$
He changes the negative fractions to positive ones by distributing the $-\frac12$ factor to each term in the parentheses; $4*2=8$ so the denominator of the fractions becomes $8$