Which solution did you expect?
You provide 3 equations with a total of 8 variables and try to solve for just one of them.
Which variables are known and which should be dependent on them?
As you provide 3 equations, it would be natural to solve for 3 variables (and assuming that they are dependent on the other 5 variables given).
Here an example where I have assumed A1 and C1 to be unknown as well - you have to solve for them even though you may not be interested in them as this is the way to tell Prime that you ant the solution to dependent on the other 5 variables.
Her another example
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