Question: Why can't I solve for a variable?

I can't believe I've just spent a half hour trying to do the simplest thing:

The second example is copied straight out of Tutorial 2.  I'm doing just what the tutorial says: right-click, Solve, Solve for Variable.  I've tried disabling everything in my startup profile, recalculating, restarting, etc.  Why in the world can't I do this simple thing?

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