Chapter 10 Counting Methods and Probability - 10.2 Use Combinations and the Binomial Theorem - 10.2 Exercises - Quiz for Lessons 10.1-10.2 - Page 697: 2

See below.

a) Using the Fundamental Counting Principle the number of possible choices is $10\cdot10\cdot10\cdot26\cdot26\cdot26=17576000$ 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 $10\cdot9\cdot8\cdot26\cdot25\cdot24=11232000$