[tex]~~~~k^2-2k-8=0\\\\\implies k^2 - 2k +1^2-1-8=0\\ \\\implies ( k-1)^2-9=0\\\\\implies (k-1)^2-3^2 =0\\\\\implies (k-1-3)(k-1+3)=0\\\\\implies (k-4)(k+2)=0\\\\\implies k=4,~~ k= -2[/tex]
