For AQA, when you get to A2, it may mention in some text books that the marginal revenue curve is the differential of the average revenue curve but it's something you really do not need to know at A2.
You have to be confident with your basic adding and subtracting as you might get a question which involves you doing a calculation like -30+-30=? and yes, this does confuse people, even when they have calculators.
Elasticity is very simple, but maths based. Essentially, you remember a few formulas and that's that.