Simple Questions, Plz solve all

Iffine Cipher: 1. Encipher "ITSCOOL" with E(x)=(5x+8)MOD26. 2. Decipher "HPCCXAQ" if the encipherment function is E(x)=(5x+8)MOD26. 3. Here is ciphertext "OYHYJLEVYQBLSRIJLYEC" that was produced using an affine cipher on the English alphabet with encryption key (5, 4). Find the decryption key and then decrypt the message. 4. Let us consider: e(x)=ax+bmod26, with a=9 and b=2. (a) Is 9 a valid choice for parameter "a"? Why? (b) Is b=2 a valid choice? Why? (c) Encrypt the plaintext "affine" (d) Find the decryption function d(y) (e) Decrypt the cyphertext "zmimcab" (f) Decrypt the cyphertext "ucr" 5. Suppose the plaintext starts with "if" and the corresponding ciphertext is "PQ". Find the key of the affine cipher. 6. Suppose you have a known plaintext situation for affine cipher. The plaintext is HAHAHA and the ciphertext is NONONO. Determine the key. 7. Suppose you have a known plaintext situation for affine cipher. The plaintext is III and the ciphertext is QQQ. Explain why this is not enough information to determine the key. 8. I decide to make affine cipher more secure by encrypting first with one key, and then encrypting again using another key. Is there any reason this is more secure? Why or why not