After executing the query, you can use mysqli::$insert_id value or mysqli_insert_id function to retrieve the last generated id, like this:
mysqli_query($con,"INSERT INTO users_accounts (business, email_uniq) VALUES ('$business', '$email_uniq')");
$insert_id = mysqli_insert_id($con);
or using the object functions:
$con->query("INSERT INTO users_accounts (business, email_uniq) VALUES ('$business', '$email_uniq')");
$insert_id = $con->insert_id;
edit: Not related, but definitly important!
If the values for either of these parameters $business or $email_uniq are user supplied, it is highly recommended to make sure they are filtered properly. The easiest way is by using a prepared statement for security (http://php.net/manual/en/mysqli.quickstart.prepared-statements.php). Here is your code using prepared statements:
$stmt = $con->prepare("INSERT INTO users_accounts (business, email_uniq) VALUES (?,?)");
$stmt->bind_param("ss", $business, $email_uniq);
$stmt->execute();
$insert_id = $con->insert_id;