How to extract key from SSL .crt OR .p7b file?

I am having SSL .crt and .p7b files and i want to extract the key (RSA) and convert them into pem file.

any one is having any idea ? i am not having self signed certificate but i am having SSL purchased certificate for my server.

key.pem and cert.pem file i want to use for nodejs HTTPS.

via Bharat Chauhan