Sorry for all the math questions but i do not understand this at all. someone explain and write the equation for me ? ? they are the exact doesn't make sense to me:

the sum of three times a number (z) and 15 is equal to 21.

Needmax recorded the heights of 500 male humans. he found that the heights were normally distributed around a mean of 177 centimeters. which statements about max’s data must be true? a) the median of max’s data is 250 b) more than half of the data points max recorded were 177 centimeters. c) a data point chosen at random is as likely to be above the mean as it is to be below the mean. d) every height within three standard deviations of the mean is equally likely to be chosen if a data point is selected at random.