ii. Calculate Pr(X < 8000) = Pr(X > 20,000) by symmetry
= 1 - Pr(X < 20,000).
iii. Calculate Pr(8000 < X < 12,000) = Pr(X < 12,000) - Pr(X < 8000).
Note that Pr(X < 12,000) = Pr(X > 16,000) by symmetry
= 1 - Pr(X < 16,000).
In each case, convert the given X values to Z values and then use your standard normal distribution tables to calculate the required probabilities. You will find many worked examples of this type in this subforum.
If you need more help, please post what you've done and where you get stuck.