Purpose is the reason you are writing the text. In this case it is to persuade your audience into thinking in a certain way. It is important to read carefully in the question as to who your audience is as this will also affect your writing style. Persuasive texts commonly use techniques such as repeated words, rhetorical questions and facts and figures.
Repeated words can be effective as they nail into the reader your message clearly and the 'Use of Three' achieves this. For example, "Safer streets means comfort, reassurance and peace of mind you for, your family and your friends." Rhetorical questions are questions that you ask without expecting an answer. The question might be one that does not have an answer. It might also be one that has an obvious answer but you have asked the question to make a point, to persuade or for literary effect. For example, "Who wouldn't want their streets to be safer?". The final technique of using facts and figures allows the audience to feel as though they can trust what you are saying or writing as it is backed up by credible data. You won't be able to access the data during your exam but it is a technique you can use for coursework and, if you want to use some during your exam, a little bit of poetic licence won't hurt anyone. For example "80% of people agree that installing more speed cameras would make their streets safer".
There are other techniques as well and this answer can be expanded upon during the session. Other types of purposes could also be addressed.