^{3}+my

^{3}-x-y,

I get this far

m(x+y)(x

^{2}+xy+y

^{2})-x-y

and then I do this

m(x+y)(x

^{2}+xy+y

^{2})-1(x+y) to get (m-1)(x+y)(x

^{2}+xy+y

^{2}) which is wrong.

I've worked out what I should have done (I think!),

(x+y)m(x

^{2}+xy+y

^{2})-x-y

(x+y)(mx

^{2}+mxy+my

^{2})-x-y

(x+y)(mx

^{2}+mxy+my

^{2})-1(x+y)

(x+y)(mx

^{2}+mxy+my

^{2}-1)

The worrying thing is, I don't know WHY I can't do what I did when I went wrong ! So how do I recognise when I'm going to do it again?