Hello help please i really need this due soon and i don’t know how

Find the value of the cosine for angle a

7x+8y=-18, 4x-9y=-3 solve the system of equations

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%