Your answer is B. 13.
In this triangle, to make sense with the law of cosines, we can say that N is A, K is B, and H is C. This means we can substitute the numbers into the formula, so:
a² = 12² + 6² - (2 × 12 × 6) × cos(90) = 180. Then, we just need to square root this as it is a², and √180 = 13.416, which rounded to a whole number is 13.
I hope this helps!
