We have seen two variants of string-merge on the accompanying slide. Both make use of (apply string-merge ...) in the cases where one of the lists become empty. apply calls a function on a list of parameters. Thus (apply + (list 1 2)) is equivalent to (+ 1 2).
Eliminate the use of apply, such that string-merge (tail) recursively builds the end of the list by a number of calls to string-append (one for each element in the non-empty list).