#4n^2-26n-42 = (2n-13/2)^2-(13/2)^2-42#
#= (2n-13/2)^2-169/4-168/4#
#= (2n-13/2)^2-337/4#
#= (2n-13/2)^2-(sqrt(337)/2)^2#
#= (2n-13/2-sqrt(337)/2)(2n-13/2+sqrt(337)/2)#
Are you sure about the #-# sign on #42#?
#4n^2-26n+42 = (2n-13/2)^2-(13/2)^2+42#
#= (2n-13/2)^2-169/4+168/4#
#= (2n-13/2)^2-1/4#
#= (2n-13/2)^2-(1/2)^2#
#= (2n-13/2-1/2)(2n-13/2+1/2)#
#= (2n-7)(2n-6)#