Multiplying each side by 3/4, we get (3/4)V=pir^3. Dividing both sides by pi, we then get r^3=(3/4)V/pi
For the second part, we get that 381.51=(4/3)πr^3, so plugging 381.51 into the equation above we get (3/4)(381.51)/pi=around 91, and r= the cube root of r^3, which would equal around 4.5 here