If "i" is a linearly incrementing counter, then all you would need is an adder I'd say.
Anticipating the next question, let me pre-emptively answer it with a question:
What are typical values of "i" you run through on say an example that runs for 100 cycles? From 0 to 99? Cool! What are A and K? Got values? excellent. Now calculate "x", and see if you see a pattern emerging here? ;-)
As already suggested, you can use a lookup table, or a multiplier, or an adder. All these can give you a solution. Which is best depends on the values of A, K and the sequence of your "i" counter. Not to mention clock speed, what parts, etc.