Hey guys could yall solve this problem for me? thanks

Me answer number 3plz explain how you do it

Find the vertex of the function given below y=x^2-6x+1

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%