HCl + HNO3 + Sn → NO2 + H2SnCl6 + H2O
NOTE: the equation is also presented unbalanced. To be of any use to a working chemist there has to be equal numbers of atoms on each side, but this is an academic setting and we are only interested in the change of oxidation states.
Of course this silly example isn't making life any easier because the tin salt is a complex ion, H2SnCl6. However, it forms the following ions:
H2SnCl6 -> 2H(+) + SnCl6(-2)
Each Chlorine atom has a charge of (-1) so ordinarily this would mean the Tin atom has a charge of (+6), because the charges have to balance. However, the ion itself has a charge of (-2), meaning the tin atom is in an oxidation state of (+8). The tin is definetly being oxidized.
2H(+) + Cl(-) + NO3(-) + Sn → NO2 + 2H(+) + SnCl6(-2) + H2O
Since each Oxygen atom has a charge of (-2), the Nitrogen in NO3(-1) has an oxidation state of (-7). The Nitrogen in the produce NO2 has an oxidation state of (-4), so the Nitrate ion is being reduced to Nitrogen dioxide gas. Hydrogen ions are forming water but they are not being reduced since the Oxygen atom has a charge of (-2), meaning the Hydrogen atoms still have an oxidation state of (+1). The only time an oxidation state is zero is when there is a pure element in the equation.
It looks as though the tin is being oxidized and the nitrogen reduced. The tin is called the oxidizing agent and the nitrogen the reducing agent. If the equation were balanced the two half reactions could be written correctly.
The rest of the homework is solved approximately the same way but frankly, this is graduate level chemistry and if it is being presented to demonstrate oxidation reduction reactions to elementary chemistry students, it is just another example of poorly taught chemistry.
BTW: I searched the CRC Handbook of Chemistry and Physics and there was no such thing as H2SnCl6 listed. It may be a totally artificial ion, but unfortunately, make believe chemicals are sometimes a part of academic chemistry.