Question 194002
Fish tank is rectangular. Adding the rocks results in a rise of 2cm.
So adding the rock displaced enough water to fill a "slice" that is 2cm x 40cm x 70cm

The volume in that slice is {{{V = L*W*H}}}
{{{V = 2*40*70}}}
{{{V = 5600}}}cubic cm

1 Liter is 1000cc, so the rock has a volume of 5.6 liters.