(a)

is the standard error, which is the standard deviation of the sample mean. (Because samples are taken randomly, the sample mean is also a random variable, so it has a standard deviation)

is the standard deviation of your population

is the number of data points in your sample

(b)

EDIT:corrected 2nd term

Sample means from a normal distribution are normally distributed. Using the information above, your sample mean follows a normal distribution

Try and use this information to answer parts (b) and (c)