What is the square root of 18 written in simplest radical form

To get the simplest radical root form for a non-perfect square, always factor out the number first.
So, for 18, find the factors.
     18
    6   3
  2  3  3
   2 3²

so, when you find a pair of the same two numbers, they become squared. Eliminate the squared signs; that number goes to the left, and the left over numbers are multiplied and are put into the square root sign. So, you end up with the final

3