1. Energy of one X-ray photon

Given wavelength: $\lambda = 10\,\text{\AA} = 10 \times 10^{-10}\,\text{m} = 10^{-9}\,\text{m}$

Energy of a photon is: $E = \frac{hc}{\lambda}$

So, $E = \frac{hc}{10^{-9}} = 10^9 hc$

2. Energy equivalent mass of fictitious particle

If a particle has the same energy, then using Einstein's relation: $E = mc^2$

Hence, $m = \frac{E}{c^2} = \frac{hc/\lambda}{c^2} = \frac{h}{\lambda c}$

Substitute $\lambda = 10^{-9}\,\text{m}$ and $c = 3 \times 10^8\,\text{m/s}$: $m = \frac{h}{(10^{-9})(3\times 10^8)}$

$m = \frac{h}{3 \times 10^{-1}} = \frac{10}{3}h$

3. Compare with given form

Given mass is: $m = \frac{x}{3}h$

So, $\frac{x}{3}h = \frac{10}{3}h$

Therefore, $x = 10$

Final Answer

$\boxed{10}$