Can someone Help me out.

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%

I’m still confused on how to work the percentages

If f(x) = -5x + 1 and g(x) = x3, what is (gºf)(0)? enter the correct answer