I have algebra students, in high school and college, who struggle with evaluating expressions like 2 – 5. This is a ubiquitous problem.

I have tried several strategies and the one that is easily the most effective is shown below. When a student is stuck on 2 – 5 the following routine plays out like this consistently.

Me: “What is 5 – 2?”

Student pauses for a moment, “3”

Me: “So what is 2 – 5?”

Student pauses, “-3?”

Me: Yes!

I implemented this approach because it ties into their prior knowledge of 5 – 2. It also prompts them to analyze the situation – do some thinking.