In order to do this, you have change the alkene into an alkyne. That is the aim of Br2/CH2Cl2 trailed by NaNH2. The Br2 with form a vic dihalide (3,4-dibromo octane). Adding of NaNH2 will execute two E2 reactions. -NH2 will eliminate an H from carbons 3 and 4. This double elimination will make the alkyne. Then handling the alkyne with H2/Lindlar will form the cis alkene. The final product will be CIS-3-octene.
