Distance = Rate * Time

That's about it. The rest is a translation exercise. Let's see if we can do it without knowing how long a Marathon is.

"Tom ran part of the 42km at an average of 10k/h"

We are not told the distance, so let's name it. The distance he ran was 'x km'. We are also not told the time, so let's name it. The time he ran for 10 k/h was "w hr".

x km = (10 km/h)*(w hr)

"walked the rest at an average 6km/h."

This time, we have a clue on the distance.

(42 km - x km) = (6 km/h)*???

We also have a clue on the time.

"he ran for one hour more then he walked."

(42 km - x km) = (6 km/h)*((w-1) hr)

"how long did he take to finish the marathon?"

We need ONLY "w" to answer the question.

Let's take a break and see if you get this far. What's next?