His monthly salary is $1500 + .065*S where S is his sales in dollars.
Remember a % is a number divided by 100. So 6.5% is 6.5/100 = .065
That is true as long as his salary is not a draw against commission. If it's a draw against commission, his pay is $1500 until his commission exceeds $1500.
For this example it's:
$1500 + .065*5000 = 1500 + 325 = $1825