Error due to code in Form Vaildation.php

Hello!

Ive looked and looked but cant seem to find how to correct issue that comes with this line of code:

 return $query->num_rows() === 0;

Ive seen various ways to correct but dont know which is right. I may have asked of this before or not but if I did please forgive me. Im trying to correct this code because its messing  everything else up   Can  someone help, THANK YOU!!!

,
Mekaboo