If the line f passes through (-4,0) and (-2,2) it means:
f(-4)=0 and f(-2)=2
If the line g passes through (-10,0) and (-8,2) it means:
g(-10)=0 and g(-8)=2
We can see that
g(-10)=0=f(-4)
g(-10)=0=f(-10+6)
Also,
g(-8)=2=f(-2)
g(-8)=2=f(-8+6)
Therefore, k=6