Symbols `._(35)^(79)Br` and `.^(79)Br` can be written whereas symbols `._(79)^(35) Br` and `._(35)Br` are not accepted. Answer in brief.

Explanation

In the symbol `._(A)^(B)X` of an element : A denotes the atomic number of the element
B denotes the mass number of the element.
The atomic number of the element can be identified from its symbol because no two elements can have the atomic number. However, the mass numbers have to be mentioned in order to identify the elements. Thus, Symbols `._(35)^(79)Br` and `.^(79) Br` are accepted because atomic number of Br will remain 35 even if not mentioned. Symbol`._(79)^(35) Br` is not accepted because atomic number of Br cannot be 79 (more than the mass number = 35). Similarly, Symbol `._(35)Br` cannot be accepted because mass number has to be mentioned. This is needed to differentiate the isotopes of an element.