I originally developed a similar activity for my math problem-solving website Math Spies. This became a paid product in my TPT store called Math Spies Coded Messages, but I wanted to develop a free version for the holidays.
Students figure out and complete the pattern for each function. They determine the equation. The y value for each part of the function is paired with a letter and so the message can be decoded. I have found that this is a motivational way to introduce and practice function tables in my classroom.