Identify the oxidizing and reducing agent in 2Ag+ + Br- _ 2Ag + Br2
The oxidation state of Ag^+ on the left is +1 and on the right it is 0. So Ag has gained an electron.
The oxidation state of Br^- on the left is -1 and on the right is 0. So Br has lost an electron.
Oxidizing agents gains electrons. Reducing agents lose electrons.