Answer:
k= 5.
Explanation:
1. Write the expression.

2. Factorize k.

3. Simplify using pythagorean identities.
Check attached image 1.

4. Verify.
Let's graph the function to see if it's true that it equals 5 for any value of x and k=5.
Check attached image 2.
As we can see in the graph, for any value of x the function returns 5. Hence, the answer is correct.