Chapter 10 Counting Methods and Probability - 10.1 Apply the counting Principles and Permutations - Guided Practice for Examples 1, 2, and 3 - Page 683: 2

See below.

a) Using the Fundamental Counting Principle and the example, the number of possible choices is $26\cdot26\cdot26\cdot10\cdot10\cdot10\cdot10=175760000$ b) Using the Fundamental Counting Principle and the example, the number of possible choices is $26\cdot25\cdot24\cdot10\cdot9\cdot8\cdot7=78624000$ (because each previously chosen letter or number cannot be chosen again).