For which pairs of functions is (fºg)(x) = x?

Can someone me with this problem i don't understand i need

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%

Which expression gives you the distance between the points (5,1)and(9,-6)