The message given has been enciphered using the formula $C=7P+17 \pmod { 26}$ where $P$ represents the letters of the alphabet taking values $a=0,\ b=1,\ {\rm to}\ z=25$ and $C$ represents the cipher value of the corresponding $P$.
It is easy to decipher the message by using the given formula to find the cipher numbers for each letter. But can you rearrange the formula to give $P$ in terms of $C$ using the multiplicative inverse of 7 (mod 26) and the additive inverse of 17 (mod 26) and hence decipher the message?