In your case, you know that
Your goal here is to determine how much heat is required in order for one mole of this substance to undergo this phase change, so you can say that
#1color(red)(cancel(color(black)("mole"))) * "28.4 kJ"/(3.21color(red)(cancel(color(black)("mole2")))) = "8.847 kJ"#
So, if one mole of this substance requires
#DeltaH_"vap" = color(green)(|bar(ul(color(white)(a/a)"8.85 kJ mol"^(-1)color(white)(a/a)|)))#
The answer is rounded to three sig figs.