What is the molar concentration of ....

What is the molar concentration of Br- ions in a 0.250 M aqueous solution of FeBr3, assuming complete dissociation?

Please explain your answer. Thank you!!

First look into what it dissociates into in solution:

FeBr3 goes to Fe+++ and 3BR-. So it will simply be 3 x 0.250 M - 0.75M.