this is just a math promblem that i need

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