Decryption function

You are provided with the following encryption function: f(x)=21x+11 [26]
First, you have to encrypt the word GOOGLE with this function.
Then, your mission is to find the decryption function g.
Finally, you have to decrypt the word GELKT.

 Details:

1. [26] means "modulo 26".
2. The decryption function will be of the following form: g(y)=ay+b[26], with a and b natural integers lower than 26, and y=f(x).
3. You will enter the answer lowercase in the form
   (crypted word)_(decryption function)_(decrypted word)
4. Example:
   If the encryption of GOOGLE was BTEVER, that the function was g(y)=15y+3[26], and that the decryption of GELKT was SUPER, then the answer would be: btever_15y+3[26]_super
5. If you don't do it with maths, make sure that your function works for any input