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Students at this stage don't know that they need to give a name (i.e. assign a variable) to the thing they are trying to find. They do this in very simple 1-step, intro problems, but not in more complex problems. I often use their flailing with the idea of the E(above) and E(below) as an opportunity to give a sermon on this topic. “Name the thing you don't know” is my mneumonic. They will have further problems with this activity because they also won't think about the fact that each of those E's has different components. You may need to help them name E(above, parallel) and E(above,perpendicular), etc. Although some of them will know enough to choose a rectangular coordinate system adapted to their surface. expand and add this to the instructor's guide when it is written (Corinne)