
Look at the graph picture I attached

c is the intercept, which is why the graph crosses the y-axis at +3
to find where it crosses the x-axis, we make y = 0

It’s i, II, and iii I do believe.
To get the difference between the two diameters, all you have to do is subtract the smaller diameter (Mercury) from the larger diameter (Saturn) as follows:
Difference between the two diameters =diameter of Saturn-diameter of Mercury
Difference between the two diameters = (1.2*10^5) - (4.9*10^3)
Difference between the two diameters = 115100 km