This answer is not quite correct. You do have the idea that the "p" in pH stands for "- log" but you forgot to take the square root of 10-10. This question is really solved by solving the equation...10-10 = [H3O]2. Remember that the square root of an exponential number is ½ of that number so the hydrogen ion concentration is equal to 10-5. Now you can apply your knowledge of the definition of "p" to get the correct answer.