#### Answer

See below.

#### Work Step by Step

a) Using the Fundamental Counting Principle, the number of possible choices is $10\cdot10\cdot10\cdot10\cdot10\cdot26\cdot26\cdot26=1757600000$
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 $10\cdot9\cdot8\cdot7\cdot6\cdot26\cdot25\cdot24=471744000$