It's probably in the hardcopySetheader.pg file for which a default might be in webwork2/conf/snippets.
You can set the default file to use for this in global.conf and override it in course.conf. The header files are chosen the Hmwk set editor "Set Detail for set ..." page near the top. I usually set both the (screen) setHeader and the hardcopy set header to the same file so that updates stay in sync. A slight downside to this is that in that case you can't use pure HTML in a screen file.
Hope this helps.