so calculate the Volume of the water that is already in the tank
so l x w x h
4 x 18 x 12
=864cm^3 is in the tank
find the total volume of the tank:
6 x 18 x 12
=1296cm^3
and subtract the current from the total to find what you need:
1296cm^3 - 864cm^3
= 432 cm^3
you can also find how much you need by using the remaining 2cm from the height (6-4=2) and use the volume formula with 2 substituted as the height and you’ll get the same answer
