Exam approach difference for P3 V P5:

Many has been said about how to approach a P5 exam and how to differentiate it from approaching P3 exam. But still i get confused of the difference. Can any body explain the difference. For instance if a question asks to "evaluate the impact of external environments on a company's performance" on a P5 exam and i need to use Porter's 5 forces..how should I approach this exam? What should be included and what shouldn't? Thanks,