The second formula is needed to show how the viscosity is measured from the graph. as V=Kt is related to Y=Mx so the gradient of the line on the graph is equal to K, which is given. From that you can work out what n (viscosity) equals.
The density of a sphere is calculated by the volume of the sphere which is 2/3 x pie x radius cubed. Then you can put this value into the equation density = mass/ volume.
Bit complicated I know but hopefully its cleared a few things up for you