diff --git a/src/extra_exercises.md b/src/extra_exercises.md
index 049c383e9ad3ca6d234625f644372c498a217b6b..3ea743d3b1b5b0e50d83e07d245a4c56e592a8bf 100644
--- a/src/extra_exercises.md
+++ b/src/extra_exercises.md
@@ -38,7 +38,7 @@ The figure below shows a conventional unit cell of one of the possible crystal s
 ![plan-view-zb](https://upload.wikimedia.org/wikipedia/commons/c/c0/Sphalerite-unit-cell-3D-balls.png)
 
 ??? info "source"
-    By Benjah-bmm27, own work, public domain, [link](https://commons.wikimedia.org/wiki/File:Sphalerite-unit-cell-3D-balls.png](https://commons.wikimedia.org/wiki/File:Sphalerite-unit-cell-3D-balls.png)
+    By Benjah-bmm27, own work, public domain, [link](https://commons.wikimedia.org/wiki/File:Sphalerite-unit-cell-3D-balls.png)
 
 1. What type of lattice does zincblende have? Give a set of primitive lattice vectors of this lattice.
 2. What is the basis of this crystal if we pick primitive lattice vectors? And what would be the basis if we pick conventional lattice vectors?