Work Shown
z = (x - mu)/sigma
-1.50 = (-12)/sigma
-1.50sigma = -12
sigma = -12/(-1.50)
sigma = 8
Note the phrasing "12 points below the mean" indicates the distance from x to mu is 12 units. We'll have x-mu = -12 to indicate below the mean. If x-mu = 12 then we'd be 12 units above the mean.
