A block of mass 2 kg moving on a horizontal surface with speed of $4 \mathrm{~ms}^{-1}$ enters a rough surface ranging from $x=0.5 \mathrm{~m}$ to $x=1.5 \mathrm{~m}$. The retarding force in this range of rough surface is related to distance by $F=-k x$ where $\mathrm{k}=12 \mathrm{Nm}^{-1}$. The speed of the block as it just crosses the rough surface will be: