So, you know that you're dealing with a number of bromine molecules and are interested in finding out what the mass of these molecules would be.
In order to be able to do that, you need to convert the number of molecules to number of moles, then use bromine's molar mass to get their mass.
A compound's molar mass tells you what the mass of one mole of molecules of that compound is. So if you know how many moles of bromine you have, you automatically know how many grams you have.
As you know, one mole of any substance contains exactly
In your case, you would have
#1.806 * 10^(23) color(red)(cancel(color(black)("molecules Br"_2))) * "1 mole Br"_2/(6.022 * 10^(23)color(red)(cancel(color(black)("molecules Br"_2)))) = "0.2999 moles Br"_2#
Now you should pull up a periodic table and look for the molar mass of elemental bromine,
The important thing now is to realize that bromine molecules are made up of two bromine atoms. This means that the molar mass of a bromine molecule will be
#M_"M" = 2 xx "79.904 g/mol" = "159.808 g/mol"#
So, if one mole of bromine molecules has a mass of
#0.2999color(red)(cancel(color(black)("moles Br"_2))) * overbrace( "159.808 g"/(1color(red)(cancel(color(black)("mole Br"_2)))))^(color(blue)("mass per mole")) = color(green)("47.93 g")#