Answer:
a) 
b) 0.27778
Step-by-step explanation:
a)
There are 26 lowercase letters and 10 digits.
The character that must be a digit can be generated in 10 different ways and the others, in 26+10=36 different ways.
Since the password is 6 characters long, by the Fundamental Rule of Counting there are
10*36*36*36*36*36 = 604,661,760
In scientific notation this number can be expressed as
b)
By the <em>Fundamental Rule of Counting</em>, there would be
different combinations of 6 characters, but only the number computed in a) are valid combinations, so the probability of generating a valid password is

if we look on one side, we already have one measurement. 4 cm. ok, that's the measurement on that side. look to the left now. that other side is also the same length, so we take that number and multiply it be 2 to get

Wait! we're not done yet. look at the top. do you see the 3? well that's the width for the top of the rectangle. the bottom must be the same to right? so we get

add both equations and you get

The answer is 14cm! Don't forget to label. ;)