Answer:  7km
Explanation:
Hi, to answer this question, first we have to add the kilometers hiked in the first 2 days. 
9km+6km= 15km 
Since the trip was 36km, we have to subtract 15 to 36: 
36-15= 21 km left 
Since they hiked an equal number of km the next 3 days, we have to divide the remaining km by 3: 
21/3 = 7km 
Feel free to ask for more if needed or if you did not understand something.