HD:
a)
FeS - 9e = Fe+3 + S+6
N+5 + 3e = N+2
------------------------------------
FeS + 3N+5 = Fe+3 + S+6 + 3N+2
FeS + 6HNO3 ---> Fe(NO3)3 + H2SO4 + 3NO + 2H2O
b)
Fe+3 + e = Fe+2
2I- -2e = I2
-------------------------
2Fe+3 + 2I- = 2Fe+2 + I2
2FeCl3 + 2KI ---> 2FeCl2 + I2 + 2KCl