You haven't provided the $ or % that he gets, but I will attempt to use algebraic terms to explain it instead.
If the salesman sells a car for $a and gets b% of the sale price as commission, then what you need to do is divide the a by 100, then multiply it by b to find out b% of a. This will be the commission that he will earn.
a / 100 x b = commission earned in $
Edit
You've now supplied me with the $ and %, let me explain it again.
To find out the commission that he will earn, you divide 8,100 by 100 then multiply it by 9.
8,100 / 100 = 81
81 x 9 = $729 commission earned.