Pardon the handwriting

This might take a while to get, but try working backwards.

2nd last sentence mentions that Na2CO3 is neutralised. This can only be done by HCl. Hence, when dissolving M2O3 in HCl, there would be an excess of HCl that is able to neutralise sodium carbonate.

So the first step you need to actually find is the moles of original HCl as well as the moles of sodium carbonate (hence moles of HCl required to neutralise.

With the moles of HCl to neutralise found, scale up the volumes to get the number of moles at 400cm3. This is now comparable to the original moles of HCl, as both are at the same volume.

Subtracting both moles of HCl gives the moles needed to dissolve M2O3. Stoichiometric ratio will give the moles of M2O3 you started. From there you can get the atomic mass of M, which is identified to be Cobalt. FYI M2O3 is cobalt (III) oxide, as it is usually in group 2 and not group 3.

Go try it out. Hope this helps!