Assuming Equal volumes and Equal temperature , pressure conditions.We can apply Avogadro Law.

ie. equal volume of gasses contain equal number of moles at same temperature and pressure.

Now, you can proceed .

Thus. $$\rho=Mass/Volume=\frac{n\times M_{molar mass}}{volume}$$. ==>$\rho1/\rho2=M1/M2$ $==>M_{gas}=10\times M_{H_2}$

Assuming equal volume $V$, equal temperature $T$, and equal pressure conditions $p$, we can apply Avogadro's Law: An equal volume of two gasses contain an equal number of molecules at the same temperature and pressure.

Now, you can proceed:

\begin{align} \rho &= \frac{\text{Mass}}{\text{Volume}}\\ &= \frac{n\times M (\text{molar mass})}{\text{Volume}}\\ \implies\frac{\rho_1}{\rho_2}&=\frac{M_1}{M_2}\\ \implies M_{\ce{X}}&=10\times M_{\ce{H2}} \end{align}