Numbers and letters

This task is about combinations.
 
Issa was arranging letters and numbers in different combinations.
She found that the letters a and b can only be combined in two ways, ab and ba,
but if there were three letters, she could combine them in six different ways.
 
a)  Write down all the different ways Issa could combine all three of the letters a, b, and c in the box below.
 
 
 
 
 
 
 
 
 
 
b)  Using a pattern Issa then wrote down all the different combinations of the numbers 1, 2, 3, and 4.
 
     Fill in the six missing combinations in the spaces below.
 
1234
 
1243
 
1324
 
1342
 
1423
 
1432
 
2134
 
2143
 
2314
 
2341
 
2413
 
2431
 
3124
 
3142
 
___________
 
___________
 
3412
 
___________
 
4123
 
4132
 
___________
 
4231
 
___________
 
___________