For most Cambridge examination questions that require you to express your answer in "the simplest form", the answer u have provided on top will not be accepted since it is an improper fraction, and can be simplified into the answer u have provided below (hence the answer below will be accepted). Do your best to form the habit of expressing the final answer in the simplest form, because there has been a long trend of Cambridge questions that require you to express them as such :)

Dear Sir, thank you so much for your prompt advice. Appreciated. :)

An exception is ‘express your answer as a single fraction in its simplest form’, where the bottom version obviously does not qualify as a valid form.

Thank you so much for your feedback, Sir
:)