Answer:
5605.01 dollars
Explanation:
Given that after 3 years your brother will pay your 7100 dollars.
If invested otherwise, interest is 8.20 percent per year
Let P be the amount invested
Then after 3 years at compound interest final amount
A =

This must be less than or equal to 7100 if willing to lend to brother
i.e.

So willing to lend upto 5605.01 dollars