To solve #x^2+sqrt3x-6=0#, there could be two ways, one, by factorizing the quadratic polynomial and two, using completing square method.
For factorizing we should split te middle term #sqrt3# in two parts whose product is #-6#. As #sqrt3#, would be involved these are #2sqrt3# and #-sqrt3#, and hence above becomes
#x^2+2sqrt3x-sqrt3x-6=0#
or #x(x+2sqrt3)-sqrt3(x+2sqrt3)=0#
i.e. #(x-sqrt3)(x+2sqrt3)=0#
i.e. #x=sqrt3# or #-2sqrt3#
Other way could be
#x^2+2xxsqrt3/2xx x-6=0#
or #(x^2+2xxsqrt3/2xx x+(sqrt3/2)^2)-(sqrt3/2)^2-6=0#
or #(x+sqrt3/2)^2-27/4=0#
or #(x+sqrt3/2)^2-((3sqrt3)/2)^2=0#
or #(x+sqrt3/2+(3sqrt3)/2)(x+sqrt3/2-(3sqrt3)/2)=0#
or #(x+2sqrt3)(x-sqrt3)=0#
i.e. #x=-2sqrt3# or #sqrt3#