$${W_e = \frac{1}{2} kx_i^2 – \frac{1}{2} kx_f^2 \\ = \frac{1}{2} k(x_i^2 – x_f^2) = \frac{1}{2}(408\, \mathrm{N/m})[(17 \times 10^{–3}\, \mathrm{m})^2 – (–12 \times 10^{–3}\, \mathrm{m})^2] \\ = 0,030\, \mathrm{J} = \underline{\underline{30\, \mathrm{mJ}}}}$$