Problem $4$: Answer the following questions about RSA.

a$.(7$ points$)$ Using Alice's RSA public key $(N,e)=(2174087,280421),$ Bob encrypts the message $m$ to obtain the ciphertext $c=1595761.$ Help Alice find the decryption key $d$ and Bob's message $m.$

b$.(3$ points$)$ Bob would like to send the message $m=1626642$ to Alice using Alice's RSA public key $(N,e)=(2174087,280421).$ Help Bob find the appropriate ciphertext to send.