diff --git a/assignments/a4/a4.tex b/assignments/a4/a4.tex
index 2a73d4a..5726d3d 100644
--- a/assignments/a4/a4.tex
+++ b/assignments/a4/a4.tex
@@ -52,8 +52,8 @@ We need to prove: $1 | (a + kn) \land 1 | n \land (\forall e \in \N, e | (a + kn
         Suppose $e | (a + kn) \land e | n$ \\
         $a + kn = ex \land n = ey$ for some $x,y \in \Z$ \\
         $a + key = ex$ for some $x,y \in \Z$ \\
-        $a = e(ky + x)$ for some $x,y \in \Z$ \\
-        Let $c = (ky + x)$ \\
+        $a = e(x - ky)$ for some $x,y \in \Z$ \\
+        Let $c = (x - ky)$ \\
         By substitution, we now have: $a = ec$ \\
         Therefore, $\exists c \in \Z$ s.t. $a = ec$ is true. \\
         Which means $e | a$ is true. \\