Cer file to .pem file with private key

Hi Everyone,

Trying to Make client SSL connection, Exported the Public key from Mongo DB and got it singed by my organization as cer file. Now if i try to use open SSL and convert this .pem file. im unable to start the mongo db services SSL authentication fails.

When i do the cat and check the .pem file i do not see the private key. How can i add the private key in cer file and convert to .pem file.

What am i doing wrong here, help appreciated.

thank you