-11F + 3.5 = -7.5F. I got 7.5 because the temperature rose by 0.5 (or half) of a degree each hour for 7 hours. The next step would be to drop the temperature by 2.25 (or 2 1/4) for each hour for 3 hours. So 2.25 x 3 = 6.75F. Then we subtract from 7.5. So -7.5 (or 7 1/2) - 6.75 (or 6 3/4) = -14.25F (or -14 1/4).
So, C. -14 1/4 is the correct answer.