First, I figured out the times. You know the total time is 18 hours, and that the second day was two hours longer, so if you let t be the time for the first day:

t+(t+2)=18
2t+2=18
2t=16
t=8 for day one and 10 for day two.

Then I set up a dist=Rate * time formula, adding the two rates and times together for the total distance.

Total distance=64 Day one distance=8r Day two distance=10(r+1)

64=8r + 10(r+1)
64=8r + 10r +10
54=18r
r=3

so 3mph