Question 13: x(a-b)+ y(b-a) = x(a-b) - y(a-b) , by taking out the negative of (b-a)
= (x-y) (a-b) , using the grouping method
So the grouping method is basically, as follows:
If you're given a(c+d) + b(c+d) , it can be "grouped" (factorised is a better...