#sqrt6+2sqrt6# can be simplified by multiplying the expression by its conjugate term which is #=color(blue)(sqrt6-2sqrt6#
#=(sqrt6+2sqrt6) * (color(blue)(sqrt6-2sqrt6))# , here we can apply the below property:
#(a+b)(a-b) = color(blue)((a^2-b^2)#
#=(sqrt6+2sqrt6) * (color(blue)(sqrt6-2sqrt6)) = (sqrt6)^2-(2sqrt6)^2#
#=6 -24#
#=color(blue)(-18#