To find the mean when you know the standard deviation, x, and z, you can use the following formula:
mean = x - (z * standard deviation)
where:
x is the value of interest
z is the number of standard deviations away from the mean
standard deviation is the measure of the amount of variation or dispersion of a set of values
By using this formula, you can find the mean of a set of values if you know the value of interest, the number of standard deviations away from the mean, and the standard deviation of the set of values !!