You need to do both: following the APA paper format and solve the exercises 3 & 5. The format is very similar to a lab report, except it’s an essay with focus on statistical/mathematical analyses. 
Points to consider are: 
summarize the whole assignment in the introduction, explain symbols or special glossaries etc. 
background details of the presented topic, techniques and concepts, etc. 
In the main part, describe the steps of how you solve the problem in greater detail: explain your thought, show the problem to the reader, and analyze the provided solution. 
Conclude the paper with either reword what you did,  what you learn, and/or what’s the implications of the concept in the paper.  
In the references section, you should definitely refer to the textbook and any book you used or cited. 
I found a useful link: 
https://www.customwritings.com/howtowrite/post/mathematics-essay-guide/