Let $E$ be an elliptic curve over $\mathbb{Q}$ with conductor $N$. Let $f=\sum a_n q^n$ be the weight 2 modular form corresponding to $E$. Define $L_2(f,s)=\zeta(s-1)L(Sym^2(E),s)$. The following identity is mentioned in Zagier's paper "Classical and alliptic polylogarithms and special values of L-series":
$L_2(f,s)=(1+N^{1-s})\zeta(2s-2)\sum_{n=1}^{\infty}\frac{a_n^2}{n^s}$.
I would like to find a proof of this identity. Thanks