As Richard suggest ask the professor for clarification that is what he/she is there for. It's hard to tell what he meant just from your question but he could mean the 3 ways you add CSS.
1. Inline
It is <b style="font-size:16px;">NOT</b> me.
2. In a page header
<head>
<title>MY CSS PAGE</title>
<style type="text/css">
.headlines, .sublines, infotext {font-face:arial; color:black; background:yellow; font-weight:bold;}
.headlines {font-size:14pt;}
.sublines {font-size:12pt;}
.infotext {font-size: 10pt;}
</style>
</head>
3. Linked CSS file
<head>
<title>MY CSS PAGE</title>
<link rel=stylesheet href="whatever.css" type="text/css">
</head>
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