Answer:
k=3
Explanation:
Since we know for a fact, thanks to it being given in your question, that the triangles are similar, that means the lengths are proportionate.
Secondly, you should notice a trend between both shapes. ΔPQRs sides are half of ΔDEFs.
With that in mind, we can conclude that for side K, it's half of 6, so 3! :D