What are the negative electrodes in these two systems?
Pt, Br^-, Br2 !! Pt, Fe^2+, Fe^3+
and
Pt, Br^-, Br2 !! Pt, I^-, I2
Look up the Br^- ==> Br2 couple.
Look up the Fe^+3 ==>Fe^+2 couple.
Make one an oxidation and one a reduction, choosing the arrangement that produces a + voltage for the cell. Then the negative electrode will be the one at which oxidation occurs.
The other one is done the same way.