I don't think decompose it the word you want.
CPhill has factorised your expression.
that is, the factors of $${{\mathtt{x}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{\mathtt{9}}$$ are $${\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{3}}$$ and $${\mathtt{x}}{\mathtt{\,\small\textbf+\,}}{\mathtt{3}}$$ so
$$x^2-9=(x-3)(x+3)$$
$$x^2-9$$ is the difference (take away) of two squares x squared and 3 squared
$${{\mathtt{x}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{\mathtt{9}} = {{\mathtt{x}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{{\mathtt{3}}}^{{\mathtt{2}}}$$ but it is quite the simplest form
I don't think decompose it the word you want.
CPhill has factorised your expression.
that is, the factors of $${{\mathtt{x}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{\mathtt{9}}$$ are $${\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{3}}$$ and $${\mathtt{x}}{\mathtt{\,\small\textbf+\,}}{\mathtt{3}}$$ so
$$x^2-9=(x-3)(x+3)$$
$$x^2-9$$ is the difference (take away) of two squares x squared and 3 squared