Subgoal *1/2
(IMPLIES (AND (NOT (ENDP A))
(EQUAL (APP (APP (CDR A) B) C)
(APP (CDR A) (APP B C))))
(EQUAL (APP (APP A B) C)
(APP A (APP B C)))).

Click on the link above (the open parenthesis before ENDP) to replace
(ENDP A) by its definition.