131=1/3*3.14*r^2*10
I assume you mean \(\pi\) by 3.14.
\(\begin{array}{rll}131 &=& \dfrac{1}{3}\cdot \pi \cdot r^2 \cdot 10\\ 393&=&10\pi r^2\\ r^2 &=& \dfrac{393}{10\pi}\\ r&=&\sqrt{\dfrac{393}{10\pi}}\end{array} \)
which is approx. 3.53688825481142
