[racket] why here 2 functions

I have to make this exercise :

Design the function different. It consumes two (representations of 
<http://www.ccs.neu.edu/home/matthias/HtDP2e/htdp2e-part1.html#%28counter._data-uni._%28exercise._struct9%29%29>) 
three-letter words and creates a word from the differences. For each 
position in the words, it uses the letter from the second word, if the 
two are the same; otherwise it uses the marker "*". Note: The problem 
statement mentions two different tasks: one concerning words and one 
concerning letters. This suggests that you design two functions.

Im my opion I only need 1 function.

I use a cond to check if the letters are the same and if print the 
letter otherwise print x .

Can anyone give me a tip what the second function can be.

Roelof

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