diff --git a/book/syntax_exercises/009.md b/book/syntax_exercises/009.md
index 6fc9fc2..c2d560e 100644
--- a/book/syntax_exercises/009.md
+++ b/book/syntax_exercises/009.md
@@ -102,7 +102,7 @@ The displacement can be found using the equation $s = v \cdot t$.
 13. For colours with hyphens we use need to use the class-based command `\class{named-colour}{...}. Try this using the format shown below:
 
 ```md
-$$ \class{hyphen-color}{\sum}_{n=1}^\infty a_n $$
+$$ \class{hyphen-color}{\sum}_{n=1}^\infty $$
 ```
 
 14. Test your understanding by creating a block equation where each term is a different colour!