Chapter 10 Counting Methods and Probability - 10.1 Apply the counting Principles and Permutations - 10.1 Exercises - Skill Practice - Page 687: 16

See below.

a) Using the Fundamental Counting Principle the number of possible choices is $26\cdot26\cdot26\cdot26\cdot26\cdot26=308915776$ b) Using the Fundamental Counting Principle and the fact that the number of possible numbers/letters decreases by $1$ if we choose a number/letter, the number of possible choices is $26\cdot25\cdot24\cdot23\cdot22\cdot21=165765600$