If the salt in the hash is known to us, then is it possible to crack to extract the password from the hash? If yes, how?
Hash functions are designed to go only one way. If you have a password, you can easily turn it into a hash, but if you have the hash, the only way to get the original password back is by brute force, trying all possible passwords to find one that would generate the hash that you have. Assuming the salt is very long, not knowing the salt would make it nearly impossible to crack (due to the additional length that the salt adds to the password), but you still have to brute force even if you do know the salt.
As an example, let's say that the password is "secret" and the salt is "535743". If the salt is simply appended to the end of the password, then the hash you'd be cracking would be a hash of the string "secret535743". Without knowing the hash, you'd have to try all possibilities until you reach "secret535743", which would take quite a while due to its length (keeping in mind that real salts are much longer than this).
But if you know that the salt is 535743 and that it is appended to the end of the password, then instead of trying everything, you'd try "a535743", "b535743", "c535743", etc. This greatly reduces the number of possibilities you have to try until you reach the correct string.
With that being said, it is generally quite rare to have a situation where you know the hash but not the salt since both are usually stored in the same place.
If you know how the salt is used when hashing the clear text phrase this only makes it easier to brute force. The number of possibilities you have to check will be going dramatically down, since you only have to check for the phrase without the salt. Still if it is a long phrase with many different characters it will take a lot of time.
When password is not salted, and for example function md5 is used, then a potential hacker can go to online databases, and just a lookup there for a right password.
When password is salted, then one must brute force it, which is very time consuming. If attacker do not know salt, then in practice it's impossible to hack it at all.