Running time of sparse matrix multiplication
The question doesn't make any sense. It doesn't make sense to ask for the running time of a computation, unless you specify an algorithm for that computation. So how would you calculate this product? That's the first thing you need to do: Specify an algorithm how you calculate the product. 

The next step: Assuming that many array elements are zero, can you reduce the amount of work that this algorithm would be doing? For example, if you know that x = 0, then it's pointless to multiply xy because you know the result is 0. And it's pointless to add xy to z, because that doesn't change z. So how can you take advantage of many array elements being zero? What's the runtime if you do that? 

The next step: Assuming that many array elements are zero, can you reduct the amount of work further? Would you need some clever data structures? If yes, which ones?