Answer
The z-scores are -1.555 and 1.555.
Work Step by Step
The middle 88% goes from the 6th percentile to the 94th percentile.
The area of the standard normal deviation to the left of the 6th percentile is equal to 0.06.
According to Table V, there are 2 z-scores which give the closest value to 0.06: -1.55 and -1.56. So, we use the mean of these z-scores:
$z=\frac{-1.55+(-1.56)}{2}=-1.555$
Due to the symmetry (both standard normal distribution and the middle 88% are symmetric), the z-score for the 94th percentile is 1.555.
