From the details provided in the question, the formula;
[tex]M=S-N[/tex]Can be used to derive either of;
Markdown, Original selling price or Reduced price.
Having been given;
[tex]\begin{gathered} \text{Markdown}=13.35 \\ \text{ Reduced Price=5.64} \\ \text{Original selling price=?} \end{gathered}[/tex]We can now substitute the known values as follows;
[tex]\begin{gathered} M=S-N \\ 13.35=S-5.64 \\ \text{Add 5.64 to both sides;} \\ 13.35+5.64=S-5.64+5.64 \\ 18.99=S \end{gathered}[/tex]The original selling price, that is N, is now;
$18.99