Hello,
You can do this in two ways :
- develop by using the identity : (a-b)²=a²-2ab+b², then multiply by (r+1). But this will make a multiplication 3 terms by 2 terms
- write :
Use the identity : (a-b)(a+b)=a²-b²
Then, it will be a multiplication 2 terms by 2 terms, which is more beautiful