Factorisation HALP

I have to factorise the term:

 

 

(ax-bx-ay+by ) / (x-y)

 

 

I know the solution is     " a - b "

 

I just don't know how to come to a solution

 

 

I tried:

 

(x (a-b) -y (a-b) ) / (x-y

 

but then it comes to 

 

(a-b) (a-b)

 

which expands to a 2nd degree equation, so I really don't know how to exaclty factorise until (a-b) is the answer.