Answer: a) 1977060966, b) 146581344.
Step-by-step explanation:
Since we have given that 
There are 10 numbers,
There are 26 lower case letters,
There are 26 upper case letters,
Since the first 3 characters have to be digits and next 6 can be upper or lower.
So, a) repetitions are allowed:

b) repetitions are not allowed:

Hence, a) 1977060966, b) 146581344.
 
        
             
        
        
        
I don't think you need answer for that you already did it.
 
        
             
        
        
        
Answer:
11
Step-by-step explanation:
<u><em>SUBTRACT</em></u>
 
        
             
        
        
        
Um idk if its right but i think 
s would be -3,6
r -5,3 
        
             
        
        
        
Answer:
n= -9
Step-by-step explanation:
-3 ( n+5 ) = 12
-3n - 15 = 12
-3n = 12+15
-3n = 27
n = -27/3
n = -9