I'm trying to follow this example of ElGamal's encryption scheme (page 2, slides 3 & 4), but don't understand this step (decryption, step 2):
$$(k^x)^{-1} \pmod m$$ Where $$k = 10,\ x = 9,\ m = 13$$
The slide shows that it becomes $$12^{-1} \pmod{13}= 12$$
Where did 12 come from? Also, can you help me see how the above equals 12?
I calculate:$$12^{-1} = \frac{1}{12}$$