This is a difference of squares. Every time you have the difference between two squares, you can factor it as the product between the sum and the difference of the roots:
In your case, the squares are
So, the difference between the squares, 
can be written as the multiplication between the sum and difference of the roots:
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
