Radius From the Volume

When a sphere's volume is known, the formula gives the radius. The last step undoes a cube, so you take a cube root: the number that, multiplied by itself three times, gives what you have.

This sphere holds 288π. The formula says $288\cdot \pi = \frac{4}{3}\cdot \pi\cdot {r}^{3}$. Cancel the π and multiply by 3/4: ${r}^{3} = 216$. Since $6\cdot 6\cdot 6 = 216$, the radius is 6.

A cube root, unlike a square root, has only one real answer, so there is no negative radius to throw away. On the board, the moves take the cube root for you once r³ stands alone.