We have to get that expression in terms of a and b,
Since a is the sum of x and y and b is the product of x and y,
we look for ways to get the sum a=x+y and the product b=xy.

Looking at this:



We notice that the first two terms factor as xy(x+y)



which immediately becomes ba 



We look at the last two terms.

Since , we notice that those last two 
terms will be  if we add 2xy between them, but 
if we do that we'll have to subtract 2xy to offset, so we 
do that:



Factor the middle three terms as .



Finally, we substitute a for x+y and b for xy, and we end up with



Edwin