If $f:M(n\times n) \to Sym(n \times n) \subset M(n\times n)$, Show $Sym(n \times n)$ is diffeomorphic to $\mathbb{R^{n(n-1)\over{2}}}$

Question

If $f:M(n\times n) \to Sym(n \times n) \subset M(n\times n)$,

Show $Sym(n \times n)$ is diffeomorphic to $\mathbb{R^{n(n-1)\over{2}}}$

Where $Sym(n \times n)$ is the set of symmetric matrices

I'm not quite sure where to start.