How hard is to figure out the ENCRYPTION key given message and clear text? How feasible is to brute force the ENCRYPTION key if you have


the clear-text message 
the encrypted-version of the same message 
you know that Rijandel is the encryption algorithm
you know that the salt is 2 bytes long
The attack you describe is called a "known-plaintext attack". It was commonly used to break older encryption schemes, most notably Enigma in the second world war.

AES/Rijndael is not susceptible to known-plaintext attacks.

Further reading:

Wikipedia: Known-plaintext attack

Why is AES resistant to known-plaintext attacks?