The answer that tells us that Papa wants to make a good impression on Sarah is that 'He slicks his hair back carefully and wears a clean shirt.''
This term is used to tell us that a person is trying to appear in a likeable way. That is they are trying to appear in a way that they would send off the right messages to the people that they are meeting. This is what we see here as Papa dresses up.
We can see how the passage described the actions that he carried out. He was said to have worn a clean shirt and done his hair carefully.
Read more on Sarah, Plain and Tall here:
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