How to get medium size attachment url?

After 5 days I search for retrieve url attachment thumbnail finally found as follow:

<?php
$thumb = $wpdb->get_var("SELECT ID FROM $wpdb->posts where post_parent= $post->ID and post_type = 'attachment'");
$thumbnail = wp_get_attachment_thumb_url($thumb);?>
echo '$thumbnail';

above code will display: https://www.mysite.com/images/thumb.jpg.

BUT similar as above:

I want the retrieve MEDIUM side image URL:

I Tried replacement : wp_get_attachment_medium_url
but contain errors as follow:
atal error: Call to undefined function wp_get_attachment_medium_url()

Please help me, TQ in advance…