Answer
The picture is attached below.
Work Step by Step
In order to convert 1-chlorobutane into $CH_{3}-CH_{2}-CH_{2}-CH_{2}-CN$ through the Sn2 mechanism, Cyanide ion acts as a nucleophile. It replaces chlorine from carbon number 1 and forms a bond with this carbon. Thus, $CH_{3}-CH_{2}-CH_{2}-CH_{2}-CN$ is formed. This reaction is shown below in the diagram attached.