Oxidation is the loss of elections and the gain of oxygen. The oxidation number is the same as the valence number, basically the value of the charge that an ion of the element has. Thats really it. Now, if the oxidation number is negative, then it can still lose electrons, thus it can reduce other elements or atoms, thus they are called reducing agents. About the paralellogram law, u dont need to know that for the IG using sin and cos...