I did it using the LaTeX button right next to the image button...
Erase the stuff that is already inside and then click the " Math " button in the top left. At the bottom of the list there is a little picture of a square root. You can just click that and then type the number that you want to go underneath between the {} s.
Also..you can copy and pase this: √[ ]
And just type what goes under between the parenthesees :)
Or.....you can just type sqrt( ) 