Explanation:
That function is a line that goes through the origin (0,0).
You only need one more point to be able to draw it.
Take x=4,5 the corresponding value of y is exactly what you want to know, the number of miles the lava moved after 4,5 hours.
So y = 6 * 4,5 = 27
Your second point is (4,5 , 27).
Now you can draw your line.