all done, last extension answered

csharp-fundamentals-strings.Main/Extension.cs

@@ -67,6 +67,9 @@ public StringBuilder two()
             sb.Append(reverseString);
             // ...AND THIS LINE
 
+            //technically this isn't a Stringbuilder method. That's because there isn't one in the documentation for Stringbuilder
+            // this could also be solved by making it into a char array and use Reverse() on that array
+
             return sb;
         }