Answer:
You can look at the charge of the molecule.
mg has a +1 charge so the oxidation numbrr for Mg = +1.
For Fe2O3 the total charge is 0. This means the charges of the Fe and the O needs to be equal. O has an oxidation number of -2. You got O3 so 3 x -2 = -6. You got Fe2 so -6 / 2 = -3. Oxidation number of O = -2 and for Fe = -3.
The if you go to the rightbof the reaction you got 3 MgO. O had an oxidation number of -2. The total charge of the molecule is 0 zero, so Mg should be +2.
Mg on the left of the equation has a oxidation number of +1, and on the right +2. The number changed, so it's a redox reaction.
