Answer
See below.
Work Step by Step
a) Using the Fundamental Counting Principle the number of possible choices is $26\cdot26\cdot26\cdot26\cdot10\cdot10\cdot10=456976000$
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 $26\cdot25\cdot24\cdot23\cdot10\cdot9\cdot8=258336000$
