A solid ball of radius $6.0\ \text{cm}$ carries a charge spread evenly through its volume. The ball is made of a material that does not change the field, so treat its inside as empty space. It is far from everything else. The energy of the ball's charge is stored in its electric field, with $\tfrac12\varepsilon_0E^2$ joules in each cubic metre, both inside the ball and outside it. Imagine a sphere of radius $r$, with the same centre as the ball. Inside that sphere lies exactly half of the total energy stored in the field. What is $r$? Give the answer in $\text{cm}$.