Consider the figure,
We have,
[tex]\begin{gathered} HK=x+6 \\ \text{HJ}=4x-6 \end{gathered}[/tex]As we can understand from the figure that,
HJ=HK+KJ and HK=KJ
[tex]\begin{gathered} HJ=HK+KJ \\ 4x-6=x+6+KJ \\ 4x-6-x-6=KJ \\ 3x-12=KJ \\ 3x-12=x+6 \\ x=9 \\ \therefore KJ=3\times9-12=15 \end{gathered}[/tex],
