#6sqrt2# is the simplest form of what?
You're asked to determine what radical term can be simplified to give
This means that you have to reverse the process you use when trying to get radical terms to their most simple form.
So, you start with
In this case, it is. you know that
#6 = sqrt(6^2) = sqrt(36)#
This means that you can write
#6sqrt(2) = sqrt(36) * sqrt(2)#
You know that
#color(blue)(sqrt(a) * sqrt(b) = sqrt(a * b))#
which means that you have
#6sqrt(2) = sqrt(36) * sqrt(2) = sqrt(36 * 2) = color(green)(sqrt(72))#