The margin of error is given by: ME = critical value*standard error. Standard error is `sigma/sqrt n` .
Here, sigma is known to be 7 ft. The number of samples is 32. This gives standard error as `7/sqrt 32 = 1.2374` . For the 95% confidence interval the critical value is 1.96. The margin of error is 1.96*1.2374 = 2.425
The margin of error is 2.425 and the confidence interval is 95%