Factorize:$\displaystyle a^{2}-\left ( b+c \right )^{2}$

I’ve expanded it to see if I can find any solution:

$\displaystyle \left ( b+c \right )^{2}=b^{2}+2bc+c^{2}$

$\displaystyle a^{2}-\left ( b^{2}+2bc+c^{2} \right ) $

$\displaystyle a^{2}- b^{2}-2bc-c^{2} \right ) $

But I can’t get any further.

What should I do now to simplify it? Please explain and show me a couple of clues or something?