A message of 4096 characters (with spaces) occupies 3.5 KB in memory. What is the cardinality

A message of 4096 characters (with spaces) occupies 3.5 KB in memory. What is the cardinality of the alphabet used to encode this message?

First you need to understand how many bits are used to encode each character of the alphabet.
3.5 KB = 3.5 x 1024 bytes = 3584 bytes (there are 1024 bytes in one kilobyte).
3584 bytes = 3584 x 8 = 28672 bits (there are 8 bits in one byte).
Let’s determine how many bits of data there are per one character of the text:
28672: 4096 = 7 bits.
With 7 bits you can encode 2 ^ 7 = 128 characters.
Answer: the cardinality of the used alphabet is 128 characters.