Group the first third and fourth terms as a square trinomial:



Factor the trinomial 



That's the difference of two squares:

[2(2m-1)-n][2(2m-1)+n]

[4m-2-n][4m-2+n]

----------------
-3xy+6+2y-9x

Notice that 6 is 3 times 2 and -9 is 3 times -3.
So rearrange the terms like this, positive ones first and
negative ones last:

6+2y-9x-3xy

Factor the first two by taking out a 2.
Factor the last two by taking out -3x

2(3+y)-3x(3+y)

Take out common factor (3+y)

(3+y)(2-3x)

Edwin