Use the Alphabet Coding Method (A =1, B = 2 $f ( x ) = x + 2$

Essay

Use the alphabet coding method (A =1, B = 2, C = 3, ... Z = 26) to
-The function defined by $f ( x ) = x + 2$ was used to encode a message as:
$$\begin{array} { l l l l l l l l l l } 15 & 3 & 22 & 10 & 9 & 7 & 16 & 11 & 23 & 21\end{array}$$
Find the inverse function and determine the message.