Two-tap voltage divider, why can't I solve for the center resistor?
You have 2 equations and 1 unknown variable R. Your system of equations is overdetermined. Just use the second equation, solving it is easier than solving the first one.

https://en.wikipedia.org/wiki/Overdetermined_system

For the record, it is sometimes useful to have an overdetermined system. One can use least-squares method, averaging, median to "improve" the measurement.

Edit : The original data does not make sense, V1 should be greater than 2.5V since R1 = R3 = 1Mohms and R2 should be greater than 0...