Almost all the acid/base stuff we do in chemistry is in water. Water itself (normally, under standard conditions) has a pH of around 7 because it dissociates into [-OH] and [H+] to an extent that each has a concentration of 1 x 10^-7, or .0000001. As you can see, a very small amount.
This question is probably throwing you off because 99% of the time you've seen acid solutions, we haven't accounted for the acidity for water. The concentration of given acid solutions tends to be so high that we can effectively disregard the amount of H+ from water because it is insignificant in comparison.
For example, the pH of a .01M HCl solution is 2 because the concentration of H+ is .01 or 1x10^-2. What about the H+ from water? As you recall from earlier, the concentration of H+ already there from water is .0000001. So .01 + .0000001 ...we're basically adding a number so small that the inclusion of H+ from water doesn't make a difference.
But when the solution is 1x10^-12 HCl, that's effectively .000000000001 H+ from acid with .0000001 H+ from water. In this case it is clear the amount from HCl is insignificant.
Long story short: for an H+ concentration of 1x10^-X, where X is a number greater than 7, account for the existing H+ from water!
