An offset calibration is inverted only after the offset is removed. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A nonzero offset must be subtracted
Value 0 gives signal 2, so the offset is 2.
b=2
Inverse calibration subtracts offset first
Unknown signal 11 minus offset 2 divided by slope 3 gives 3.