D. 3 √5
You do rise over run so with points F and C you go up 3 and over 6
You then square them to get 3^2 and 6^2. If you solve that you get 9 and 36. Add them together and you would get 45. You then do √ (square root) of 45. If you simplify this you would get a 5, 3, and 3. Since there are 2 3’s you put them on the outside so your answer is 3√5