Given, \[{\text{B}}{{\text{r}}^{\text{ - }}}\] and \[{\text{Br}}{{\text{O}}_{\text{x}}}^{\text{ - }}\] in acidic medium forms \[{\text{B}}{{\text{r}}_{\text{2}}}\], where \[{\text{x = }}\]

Hint: We have to know that the symbol of bromine is \[{\text{Br}}\]. Bromine is one of the halogen groups in the periodic table. The atomic number of bromine is \[35\]. The mass number of Oxygen is \[79\].The oxidation reaction means the addition of oxygen or the removal of hydrogen or the loss of electrons in the reaction from reactant to product. The reduction reaction means the addition of hydrogen or the removal of oxygen or the gain of electrons in the reaction from reactant to product.

Complete answer:
The given data is,
\[{\text{B}}{{\text{r}}^{\text{ - }}}\] and \[{\text{Br}}{{\text{O}}_{\text{x}}}^{\text{ - }}\] in acidic medium forms \[{\text{B}}{{\text{r}}_{\text{2}}}\].
If we want value of x means first balance the given reaction,
The reaction for the above statement is,
 \[{\text{5B}}{{\text{r}}^{\text{ - }}}{\text{ + Br}}{{\text{O}}_{\text{3}}}^{\text{ - }}{\text{ + 6}}{{\text{H}}^{\text{ + }}} \to {\text{3B}}{{\text{r}}_{\text{2}}}{\text{ + 3}}{{\text{H}}_{\text{2}}}{\text{O}}\]
Above reaction is a balanced chemical reaction. From this balanced chemical reaction, we found the initial oxidation state of \[{\text{B}}{{\text{r}}^{\text{ - }}}\] is \[ - 1\] and the initial oxidation state of \[{\text{Br}}{{\text{O}}_{\text{x}}}^{\text{ - }}\] is \[ + 5\]. The final product oxidation state of \[{\text{B}}{{\text{r}}_{\text{2}}}\] is \[0\]. In one reaction, oxidation and reduction happens means that reaction is called a redox reaction.
Now, we compare \[{\text{Br}}{{\text{O}}_{\text{x}}}^{\text{ - }}\] with \[{\text{Br}}{{\text{O}}_3}^{\text{ - }}\]. Hence, the value of \[{\text{x = 3}}{\text{.}}\]

Note:
We also know that in chemistry, periodic tables play a vital role. In the periodic table there are totally \[118\] elements. In the periodic table there are totally \[18\] columns and \[7\] rows. The columns are called groups. Hence, \[18\] groups in the periodic table. The rows are called periods. Hence, totally \[7\] period in the table. The symbol of oxygen is \[{\text{O}}\].Oxygen is one of the sixteen groups in the periodic table. The atomic number of Oxygen is\[8\]. The mass number of Oxygen is\[16\]. The valency of Oxygen is \[2\]. The stable oxidation state of Oxygen is \[ - 2\]. The atomic number of the element is nothing but the number of electrons or number of protons. The mass number of the atom is nothing but the sum of the number of protons and number of neutrons.