The question is very simple:
Find a prime divisor of $\frac{(10^{13}-1)}{9}$ , i.e. $11\cdots11$($13$ ones), also known as $R^{(10)}_{13}$ or $R_{13}$. Same question for $R_{79}$.
Of course, calculating the answer using a calculator is simple, but I have no idea how to tackle it. Furthermore, we know by Fermatss little theorem that $10^{12}=1 \pmod{13}$, but I can't seem to apply this to this problem. Thanks in advance.