You may find the Lewis dot structure of the CH₃-Br in the attached picture.
Explanation:
In the Lewis dot structure we represent the unpaired electrons using dots.
Bromide have one electron shared with one electron from the carbon to form a covalent bond, while the remaining electrons remains unpaired. We represent the six unpaired electrons of the bromide with dots.
You subtract the atomic number from the mass number to find the number of neutrons. If the atom is neutral, the number of electrons will be equal to the number of protons.