diff --git a/07-Kalman-Filter-Math.ipynb b/07-Kalman-Filter-Math.ipynb
index 8dbbb2c..1e57e6e 100644
--- a/07-Kalman-Filter-Math.ipynb
+++ b/07-Kalman-Filter-Math.ipynb
@@ -642,7 +642,7 @@
     "$$\\begin{aligned}\n",
     "x_1 &= x \\\\\n",
     "x_2 &= \\dot x_1 \\\\\n",
-    "\\dot x_2 &= \\dot x_1 = \\ddot x\n",
+    "\\dot x_2 &= \\ddot x_1 = \\ddot x\n",
     "\\end{aligned}$$\n",
     "\n",
     "As is common I dropped the $(t)$ for notational convenience. This gives the equation\n",