A hint to get started
Use the standard normal model directly
The source labels this again as Example 4 (Self-directed Learning). For , find (i) and (ii) a such that .
CJC · Lecture · Example 4
Identify the mean and take the square root of the variance.
Convert the event to an interval and sketch the shaded area.
Use normalcdf with lower bound, upper bound, mean and standard deviation in that order.