There are 52 cards. Of this, there are 4 aces (one of each of the four suits) and 4 kings. There are 4+4 = 8 cards that are either an ace or a king (but not both). Note that the events "drawing an ace" and "drawing a king" are mutually exclusive. Being mutually exclusive means its impossible to have both events occur simultaneously for any single card.
So there are 8 cards we want (ace or king) out of 52 total
Dividing the two values, and reducing fully, gives:
8/52 = (4*2)/(4*13) = 2/13
The final answer as a fraction is 2/13