3. A cryptanalyst has determined that the two most common digraphs in a ciphertext message are RH and NI and guesses that these ciphertext digraphs correspond to the two most common diagraphs in English text, TH and HE. If the plaintext was enciphered using a Hill digraphic cipher described by
$$\begin{array}{l}
C_{1} \equiv a P_{1}+b P_{2} \quad(\bmod 26) \\
C_{2} \equiv c P_{1}+d P_{2} \quad(\bmod 26)
\end{array}$$
what are $a, b, c$, and $d$ ?
3. $\left(\begin{array}{cc}3 & 24 \\ 24 & 25\end{array}\right)$