E_{\mathrm{pot}} = K_{f} = \dfrac{1}{2} m_{1} v_{1f}^{2} + \dfrac{1}{2} m_{2} v_{2f}^{2}

m_{1} = 0.20\ \mathrm{kg},\ v_{1f} = 12.0\ \mathrm{m/s}.

m_{2} = 0.80\ \mathrm{kg},\ v_{2f} = 3.00\ \mathrm{m/s}.

K_{f}=\dfrac{1}{2}(0.20)(12.0)^{2}+\dfrac{1}{2}(0.80)(3.00)^{2}=14.4\ \mathrm{J}+3.6\ \mathrm{J}=18.0\ \mathrm{J}