Answer
a) $6,760,000$
b) $3,407,040$
c) $3,276,000$
Work Step by Step
Note that there is 26 letters and 10 digits
a) $26\times26\times10\times10\times10\times10=6,760,000$
The numbers stay at 26 and 10 because the letters and digits may be repeated
b) $26\times26\times10\times9\times8\times7=3,407,040$
The 26 stays constant but the 10 decreases because the digits can not be repeated
c) $26\times25\times10\times9\times8\times7=3,276,000$
Both the 26 and 10 decrease because the letters and digits cannot be repeated
