Answer:
a)  
And we can find this probability using the normal standard table and we got:
 
b)  
And we can find this probability using the complement rule and the  normal standard table and we got:
 
Step-by-step explanation:
Previous concepts
Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".
The Z-score is "a numerical measurement used in statistics of a value's relationship to the mean (average) of a group of values, measured in terms of standard deviations from the mean".  
Solution to the problem
Let X the random variable that represent the variable of interest of a population, and for this case we know the distribution for X is given by:
 
  
Where  and
 and  
Part a
We are interested on this probability
 
And the best way to solve this problem is using the normal standard distribution and the z score given by:
 
If we apply this formula to our probability we got this:
 
And we can find this probability using the normal standard table and we got:
 
Part b
We are interested on this probability
 
And the best way to solve this problem is using the normal standard distribution and the z score given by:
 
If we apply this formula to our probability we got this:
 
And we can find this probability using the complement rule and the  normal standard table and we got:
