$\left\lbrace\begin{array}{rrrrrrrr}& \,x&- &4 \,y&+& 8 \, z&=&-3\\& \,x&- &7 \,y&- & \,z&=&-15\\& 3 \,x&- &15 \,y&+& 15 \, z&=&k\\ \end{array}\right.$
In order for the system of equations above to be a consistent system, $k$ must be equal to .