Answer: Hello mate!
our equation is 18√5−12√5 and we want to simplify it.
Simplification is triyng to "eliminate" all the roots (if possible). In our equation we have a square root of 5, wich is a problem, but let's se wat we can do:
18√5−12√5
= (18 - 12)√5
= 6√5
and we can not simplify furthermore, because the √5 cant be simplifyed.