Researchers from the United Kingdom studied the effect of two breathing frequencies on performance times and on several physiological parameters in front crawl swimming. The breathing frequencies were one breath every second stroke (B2) and one breath every fourth stroke (B4). Subjects were 14 male collegiate swimmers. Each subject swam 200 meters using each breathing frequency: once with breathing frequency B2, and once on a different day with breathing frequency B4. A paper states that the results are expressed as mean plus or minus the standard deviation. One result reported in the paper states that the immediate post exercise heart rate for subjects when using breathing frequency B2 was 156 ± 19 beats per minute. What are x and the standard error of the mean for these subjects