Check out [the detailed writeup](https://rootfs.eu/codeblue2017-common-modulus/)
Same as the Common Modulus 1 task, we calculate $c_1^{x}* c_2^{y} \mod n$, which gives us $m^3 \mod n$ where $m$ is the flag that we're looking for.
To recover the flag, we can calculate the cubic root of $m^3$ because it would still be smaller than $n$.