A prescriptive grammar gives rules for usage that it believes should used and deviation from the rules is wrong. As an example, a prescriptive rule for the second conditional would call form the use of the past subjunctive, so 'If I were' is the correct form and 'If I was' is an error.
A descriptive grammar looks at the way people speak and write and works out its rules from usage, rather than trying to tell people how to speak and write. Therefore, a descriptive grammar would not say that 'If I was' is wrong because it is commonly used.
I don't really understand your task, but I suppose you're being asked to write something in formal language and something in informal. However, this has little to do with descriptive and prescriptive grammar- descriptive grammar does not only look at informal and colloquial language, but it does not seek to judge informal language by the rules of formal language.
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