When you have triangles do you have to add to get you other angle or subtract

Me find the measure to side a and the process to solve the problem you appreciate it : )

Acar travels at an average speed of 52 miles per hour. how long does it take to travel 299 miles

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%