I believe instead of sum, it should be product. A product is a result of a multiplication problem, which is what we're doing when we do something "times" something else.
You could solve this by simply dividing 24 by 3 to get 8. You could then check this by multiply 3 by 8 to get 24 again.
Standard Deviation is square root of the variance. So standard deviation = 11.6 Standard Error of the mean = Standard Deviation / square root of sample size = 11.6 / 8.06 = 1.44