7. (2+sqrt(6))(2-sqrt(6)) = 4 - 6 = -2.
2*sqrt(3)*sqrt(12) = sqrt(3)*sqrt(3*4) = sqrt(3)*sqrt(3)*2 = 3*2 = 6
So column B is the greater
The others are similar to column B in this question, the numbers under
the root signs are reduced to their prime factorisations, the factors that
appear twice are brought out from under the root sign, and whatever can
be cancelled is cancelled.
RonL