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I loaded your sheet in Prime 6 and experienced the same problem.
The cause is, as so often in Prime when using the symbolics, the infamous auto-labelling "feature".
Sometimes checking the option "Units/Constants in symbolic mathematics" helps, when using units, but most of the time it doesn't. On the other hand I think that it won't hurt to let this option enables by default.
In your case the problem can be solved by manually labelling the "x" after "coeffs," to be a "variable".
Change above unit labels cm as unit only, not "auto".
Did
Still get:
I loaded your sheet in Prime 6 and experienced the same problem.
The cause is, as so often in Prime when using the symbolics, the infamous auto-labelling "feature".
Sometimes checking the option "Units/Constants in symbolic mathematics" helps, when using units, but most of the time it doesn't. On the other hand I think that it won't hurt to let this option enables by default.
In your case the problem can be solved by manually labelling the "x" after "coeffs," to be a "variable".