I have tried to understand the last step in this question, but I don't quite see the logic
Factor $$ x^2+4x+4-9y^2$$
The first thing I thought was that I could factor by grouping, however this is not successful. So, the next thing I tried to do was factor the
$$ x^2+4x+4$$ part to get $$ (x+2)^2-9y^2$$
Then factor out the common terms in the y-portion:
$$ (x+2)^2-(3y)^2$$ But this is as far as I get. I eventually looked up the answer:
$$(x+2-3y)(x+2+3y)$$Is this some difference distributive rule?