question on how to decrypt the message A message is encrypted using an affine cryptosystem in which plaintext
uses the 26 letters A through Z (all blanks are omitted), the letters are
identified with the residue classes of integers (mod 26) in the natural
way, and the encryption and decryption functions are defined by x ->
f(x) = ax + b (mod 26) and y -> g(y) = cx + d (mod 26) respectively,
where a; b; c; d are certain constants (unknown to you) and g(f(x)) is
the identity function. Suppose you intercept the ciphertext
UWWPUWKPPFWPWKPJWW:
Assuming that E and then T are the two most commonly occurring
letters in English text, find the decryption function g(y) and decrypt
the message.
This is my homework question. I really dont know how to solve it . Could anyone give me a help ? 
Thanks in advance.
 A: HINTS: First make a frequency table; this should be your first step in dealing any cipher.
$$\begin{array}{rcc}
\text{Letter}:&\text{F}&\text{J}&\text{K}&\text{P}&\text{U}&\text{W}\\
\text{Frequency}:&1&1&2&5&2&7
\end{array}$$
You’re told to assume that ‘E and then T are the two most commonly occurring letters in English text’. In other words, you’re told to assume that the most frequent letter in the message represents E, and the second-most frequent letter represents T. What are those letters?
Let’s say that you decide that the ciphertext letters representing E and T are letters $m$ and $n$, respectively, modulo $26$. E and T are $5$ and $20$, respectively, so if $f(x)=ax+b$ and $g(x)=cx+d$ are your enciphering and deciphering functions, you now know that
$$\left\{\begin{align*}
&5a+b\bmod 26=m\\
&20a+b\bmod 26=n
\end{align*}\right.\tag{1}$$
and
$$\left\{\begin{align*}
&cm+d\bmod 26=5\\
&cn+d\bmod 26=20
\end{align*}\right.\tag{2}$$
Your task, once you’ve identified $m$ and $n$, is to solve $(1)$ for $a$ and $b$ and $(2)$ for $c$ and $d$ to reconstruct the enciphering and deciphering functions.
