1)Х^2-11х+28=х^2-4х-7х+28=
=(х^2-4х)-(7х-28)=х(х-4)-7(х-4)=(х-4)(х-7)
2)Х^2-6х-7=х^2-х+7х-7=(х^2-х)+(7х-7)=
=х(х-1)+7(х-1)=(х-1)(х+7)
3)Х^2-10х+21=х^2-7х-3х+21=
=(х^2-7х)-(3х-21)=х(х-7)-3(х-7)=(х-7)(х-3)
4)5х^2+17х+6=5х^2+15х+2х+6=
=(5х^2+15х)+(2х+6)=5х(х+3)+2(х+3)=
=(х+3)(5х+2)
5)4х^2-24х+27=4х^2-6х-18х+27=
=(4х^2-6х)-(18х-27)=2х(2х-3)-9(2х-3)=
=(2х-3)(2х-9)