We know that density = Mass/Volume
Volume = Mass/density
Here mass = 7.65 g
density = 12.6 g/ml
So volume 7.65/12.6=0.607 ml =

Volume of cube
, where a is the edge length
In this case

So the length of the side of a cube having a density of 12.6 g/ml and a mass of 7.65 g is 0.848 cm.