Hi,
Am not quite sure what it is You are not understanding. z = 
Here is the standard normal curve. z=values of 0,±1, ±2,±3 shown
for ex: 50% of the area under the normal curve is to the left and right of z=0
one standard deviation from the mean(z= ±1) accounts for about 68.2% of the set
two standard deviations from the mean(z= ±2) account for about 95.4%
and three standard deviations from the mean(z= ±2) account for about 99.7%.

(A) P(z > 0.90) = 1 -P(z ≤ .90) = 1- NORMSDIST(.90) = 1-.8159 = .1841 or 18.41%
18.41% is the % of the Area to the right of the blue line

(B) P(-1.17 < z < 0) = .50 - NORMSDIST(-1.17)= .50-.121
(C) P(-0.58 < z < 1.57) = NORMSDIST(1.57) - NORMSDIST(-.58) = .9418-.2801
Here, the Excel function NORMSDIST is being used for the z-values.