If you have time, grab an A-level text book or just spend an hour or so on some A-level revision sites.
For goodness sake, don't do the first 20 minutes of a 2 hour lesson.
Or the first 20 minutes you would routinely do with any new class.
I expect you to have good enough Internet skills to have tracked down the specification, past papers and so on.
I expect you to have swotted up on what you're teaching.
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