3no2− + 8h+ + cr2o72− → 3no3− +2cr3+ + 4h2o classify each reactant as the reducing agent, oxidizing agent, or neither.

According to this reaction:
3NO2^-  +  8 H^+  +  Cr2O7^-2 → 3NO3^- +  2Cr^+3 + 4 H2O
(1) 3 NO2^-  → 3NO3^-, The oxidation state of nitrogen converted from +3 to +5 
i.e oxidation process, ∴ NO2^-  is reducing agent.
(2) Cr2O7^-2 → 2Cr^+3, The oxidation state of Cr converted from +6 to +3
i.e reduction process ∴ Cr2O7^-2 is an Oxidizing agent. 
(3) H^+ → H2O, the oxidation state of Hydrogen is still +1, i.e No change (neither)