That's one long, slow trip!
The important thing to realize is that he drives the same distance twice. Let's call the distance D. If you travel 70 miles at 35 miles per hour, you have travelled for 70 miles / 35 miles per hour = 2 hours. So travelling D miles takes (D / 35) hours. Likewise, travelling back takes (D / 25) hours.
You are told that the whole trip is 6 hours (apparently the man just enjoys driving and didn't actually spend any time in the town he spent so long driving to). So that means that if you add these up, you get 6:
(D / 35) + (D / 25) = 6
Does this now look like an algebra problem you can solve? The question is asking you to find D.
Stephen La Rocque.>