What is the value of x?

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The first-serve percentage of a tennis player in a match is normally distributed with a standard deviation of 4.3%. if a sample of 15 random matches of the player is taken, the mean first-serve percentage is found to be 26.4%. what is the margin of error of the sample mean? a. 0.086% b. 0.533% c. 1.11% d. 2.22%

[10 points, algebra 2-simplifying complex fractions]according to my answer key the restrictions are -1 and 4. but why not positive 1?