A soap bubble is blown to a diameter of \( 7 \) \( \text{cm} \). \( 36960 \) \( \text{erg} \) of work is done in blowing it further. If surface tension of soap solution is \( 40 \) \( \text{dyne/cm} \) then the new radius is _________ \( \text{cm} \). Take \( \left( \pi = \frac{22}{7} \right) \).