Actually, I don't think they yield the same answer?
Let's say for both functions that x=1, where modulus = "%".
(-9*1+12) % 26 = 3%26 = 3
(11*1-170) % 26 = -159%26 = 23
The variable "b" is the magnitude of shift in your cipher, so shifting 170 is equivalent to 14 since there are only 26 letters. It doesn't make much sense to shift beyond 25 as it just requires more work to encrypt and decrypt and does not add any additional cryptographic security. Since you always mod by 26 I don't see why you can't have b>25, it just seems redundant? (unless there's some mathematical principle that requires this to be true, but all I can tell is that a must be relatively prime to m or else you cannot decrypt) Hope that helps.