$${d = v \sqrt{\frac{m}{k}} = (0,50\, \mathrm{m/s}) \sqrt{\frac{0,40\, \mathrm{kg}}{750\, \mathrm{N/m}}} \\ = 1,2 \times 10^{-2}\, \mathrm{m} = \underline{\underline{1,2\, \mathrm{cm}}}}$$