-12 since she started 2 feet above the surface of the water than she jumped down to 10 feet under the water's surface. So it would be -12 for the first answer.
So if she was snorkeling at -12 surface of the water and she went to a rock that is 15 above her from before, it means that 15 + -12 which is 3 feet from the surface.
Hope you understand!