When exposed to light, a metal salt MX splits up to form metal M and a gas X₂. Metal M is used in making ornaments, whereas gas X₂ is used in making bleaching powder. The salt MX is itself used in black and white photography. The metal salt MX is:

Metal M is widely used in ornament making, so the metal M is silver (Ag), and its salt MX is used in black and white photography. The ${\mathrm{X}}_2$ gas is used in making bleaching powder, so the gas ${\mathrm{X}}_2$ is chlorine $\left({\mathrm{Cl}}_2\right)$. 

So, if the salt is silver and the gas is chlorine; they combine to form the salt silver chloride. Silver chloride undergoes decomposition to give silver and chlorine gas.
The reaction will be as follows:
$2\mathrm{AgCl}\left(\mathrm{s}\right) \to 2\mathrm{Ag}\left(\mathrm{s}\right) + {\mathrm{Cl}}_2\left(\mathrm{g}\right)$

The salt MX would be silver chloride (AgCl).