Description: Equations are about relationships (no, not girlfriends and boyfriends!) between the two sides of the equation. Let's again use a scale example to help us understand. Created by Sal Khan. I now want to refigure out what this mystery mass is, but we're going to start using a little bit more of mathematics. And I'll represent that mystery mass by the question mark right over here. But that's not the only thing that we have on the left-hand side.*Teacher Advisor is 100% free.