Derivation

list      list + digit

list - digit + digit

digit - digit + digit

9 - digit + digit

9 - 5 + digit

9 - 5 + 2

Therefore, the string 9-5+2 belongs to the language specified by the grammar.

The name context free comes from the fact that use of a production X . does not depend on the context of X

For example, consider the string 9 - 5 + 2 . It can be derived as:

list list + digit list - digit + digit digit - digit + digit 9 - digit + digit 9 - 5 + digit 9 - 5 + 2

It would be interesting to know that the name context free grammar comes from the fact that use of a production X . does not depend on the context of X.