Why is the average energy corresponding to a vibrational degree of freedom double that of a translational degree of freedom

According to the law of equipartition of energy, every degree of freedom has average enerrgy ofper molecule.

There are 3 degrees of freedom corresponding to translational motion, 3 corresponding to vibrational motion.

So the average energy corresponding to transational motion only is

When we also consider the vibrational motion of the molecule, the body has both transalational as well as vibrational motion, and each type of motion contributes 3 degrees of freedom.

Hence average energy per molecule when vibration is considered(in addiition to translation) is