If the lines $2x+3y+12=0,x-y+k=0$ are conjugate with respect to the parabola $y^2=8x,$ then $k=$

Question

$$\begin {array} {1 1} (1)\;10 & \quad (2)\;\frac{7}{2} \\ (3)\;-12 & \quad (4)\;-2 \end {array}$$