[raghavgarg]

If you are choosing OOP style for mysqli query then you have to use mysqli object to use any method/property of mysqli.

TRY:
$rowsAffected = $mysqli->affected_rows;

affected_rows is a property of mysqli object and not query. You can refer http://php.net/manual/en/mysqli.affected-rows.php for more information.

That looks like how I am already doing it. I've tried it with and without those parenthesis on the end.

Sorry, if I was not clear. I wasn't only talking about parenthesis, my main emphasis was on the variable you are using before that

You are using 
$q = $mysqli->query($sql);
$rowsAffected = $q->affected_rows();
                ^^
I am talking about this variable, this should be "$mysqli" and not "$q" because the property you want to use is of the "mysqli" object.

The final code should be like:
$mysqli = new mysqli($server, $user, $password, $db_name);
$q = $mysqli->query($sql);
$rowsAffected = $mysqli->affected_rows;
echo $rowsAffected;