Calculate the proportion (%) of each element in the FeBr2 substance.

We will start from the molar mass of the substance (the molar mass is found according to the periodic table by adding up all the atomic masses of the components of a given substance).
Molar mass of iron: 56.
Molar mass of bromine: 80. But since there are two bromine atoms in the substance: 80 * 2 = 160.
Iron bromide mass: 56 + 160 = 216.
Now the molar mass of iron bromide is designated as 100%. The mass of iron, respectively, for X, then the mass fraction of iron is: 56 * 100/216 = 26%.
And the mass fraction of bromine is equal to: 100-26 = 74%.
Answer: the mass fraction of bromine is 74%, the mass fraction of iron is 26%.