Put this on a graph (I recommend putting the coordinates into desmos for easy graphing)
0 hours: (0, 0)
1 hours: (1, 7)
2 hours: (2, 14)
3 hours: (3, 21)
4 hours: (4, 28)
5 hours: (5, 35)
6 hours: (6, 42)
6.5 hours: (6.5, 45.5)
After 6.5 hours the lava would have travelled approximately 45.5 miles
I hope this helps you!