My class had dome some substitution with positive integers last year. However, when we started to work with negatives some issues were arising. The page in our department booklet https://startingpointsmaths.com/curriculum-booklets/ Algebra 2, moved a little too fast for them. This task was the result of that. I’ve tried to layer up the complexity in each subsection of the task. It’s all about procedural fluency and encountering lots of common situations.