Why $(x-5)^2-4$ can be factorised as $(x-5-2)(x-5+2)$ I would like to understand why $(x-5)^2-4$ can be factorised as $(x-5-2)(x-5+2)$
I am particularly concerned with the term, $-4$.

Here is another way to look at it: Use the FOIL method on the right hand side.
$(x-5-2)(x-5+2)$ becomes $x^2-5x+2x-5x+25-10-2x+10-4$ 
Combine and cancel your like terms and you're left with $x^2-10x+25-4$, which of course can be rewritten as $(x-5)^2-4$.
