[tex]1)\\(2x+1)(x+7)=2x^2+14x+x+7=2x^2+15x+7\ \ \Rightarrow\ \ k=15\\\\2)\\(2x-1)(x-7)=2x^2-14x-x+7=2x^2-15x+7\ \ \Rightarrow\ \ k=-15\\\\3)\\(2x+7)(x+1)=2x^2+2x+7x+7=2x^2+9x+7\ \ \Rightarrow\ \ k=9\\\\4)\\(2x-7)(x-1)=2x^2-2x-7x+7=2x^2-9x+7\ \ \Rightarrow\ \ k=-9\\\\\\Ans.\ there\ are\ four\ solutions:\ k\in \{15;-15;9;-9\}[/tex]