An ideal transformer trades voltage and current while preserving power. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
An ideal transformer follows the turn ratio
Primary turns 3 and secondary turns 6 double the voltage from 6 to 12 volts.
12/6=6/3
Current trades so ideal power stays equal
Primary current 2 amperes and secondary current 1 ampere both give power 12 watts.