Since the tank is 66.06% full, it is (100 - 66.06) = 33.94% empty.
Its capacity is 6,000L. So, in order to fill it to capacity, the service station
must order 33.94% of 6,000L .
33.94% = 0.3394
"of" means "times"
0.3394 x 6,000L = 2,036.4 liters needed, to fill the tank to the rim.