added links to earlier exercises

Exercises/ex2_6.md

@@ -142,7 +142,8 @@ Or, if you wanted to read the CTA data:
 
 ## (c) Memory Revisited
 
-In the CTA bus data, we determined that there were 181 unique bus routes.
+In [Exercise 2.1](ex2_1.md) we explored memory use and in [Exercise 2.2](ex2_2.md), we
+determined that there were 181 unique bus routes in Chicago.
 
 ```python
 >>> routes = { row['route'] for row in rows }