diff --git a/src/solutions/7_Tight-binding_model.md b/src/solutions/7_Tight-binding_model.md
index 8af32d383e44a1f1f7a82287481eab060426c912..a8eb972c87fe3a18336393fc807212ea9d2fec7d 100644
--- a/src/solutions/7_Tight-binding_model.md
+++ b/src/solutions/7_Tight-binding_model.md
@@ -3,3 +3,4 @@
 ### Exercise 1: Lattice vibrations
 
 1.
+Hint: Normal modes have the same function as $$\mathbf{e}_1,\mathbf{e}_2,\mathbf{e}_3$$ in $$\mathbb{R}$$.