A little help understanding some basic algebra(I think) in conversion pathways
Yes, 3.18cm has to be raised to power 3.
$$Density=Mass/Volume$$ or upon rearrangement
$$Mass=Density*Volume$$
The information is provided about edge of the cube made of Osmium and its density. So in order to obtain mass of the cube, it is necessary to find the volume of the cube(since the information about density is supplied). The volume of the cube is edge raised to the power 3,i.e.,
$$Volume of cube=edge^3$$
Since the information regarding edge is provided in inches it has to be converted into cm because the units of density is provided in $g/cm^{3}$.
Once the the volume is also obtained in $cm^{3}$, the mass can be obtained in g.

Follow these steps:


Convert the length into same unit as the the length unit in density(here it is $cm^{3}$). So, 1.25 in=3.18cm
Now to obtain volume use the formula for volume,i.e., to cube the length. So, volume=$$(3.18cm)^{3}\approx 32.16cm^{3}$$
Multiply volume with density to obtain mass. $$Mass=32.16cm^{3}*22.59g/cm^{3}\approx723g$$