It's often much easier to work out what a passage makes you feel than how it has that effect on you, it's reader. An easy structure to remember to help you with this is P-E-E (Point-Example-Explain).
a) H2SO4 ANSWER: H: +1 S: +6 O: -2 b) N2 ANSWER: N: 0 (all elements not within a compound have an oxidation number of 0 eg I2, O2, K) c) NH4