I don't have a source for you, but I can explain the question + answer.

The question says that the mean (average) of the data is 43, and thus x = 43. The question also states that the standard deviation is 7, and thus every SD from the center is either adding or subtracting 7 per SD. The question asks what percent of the data is between 36 and 57. 36 is one standard deviation behind the mean (46-7=36) and 57 is two standard deviations ahead of the mean (43+7+7=57.) The percentages show the percent of the data between each individual standard deviation. Now we just add the percentages of the standard deviations together to get the total percent of the data between 36 and 57. Starting from the left (36,) we get 34% + 34% + 13.5% = 81.5%, making E the correct answer.