A launch-power-comes-from-voltage-times-current scan reuses the same launched voltage and current to check that the forward power P = V * I, here 12 V times 3 A giving 36 W. The arithmetic is exact only for the stated voltages, currents, powers, and rows in the ideal uniform-line, characteristic-impedance model. Real lines add loss, dispersion, connector mismatch, source impedance, finite rise time, probe/calibration limits, finite precision, and measurement uncertainty.
Power is tied to the same source-bound voltage and current. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Power uses the same launched current
The current is still 3 amperes; it is not a separate caption.
I+=3A
Launched power is voltage times current
The launched power is 12 volts times 3 amperes, or 36 watts.