From the table, you know that Jupiter is further away from the sun. So if you subtract the distance of the shorter one, which is Mars, from the distance of the longer one, which is Jupiter, you woould get the distance of Jupiter from Mars.
4.84 x 10^8 - 1.42 x 10^8 = (4.84 - 1.42) x 10^8 = 3.42 x 10^8
Therefore the distance is 3.42 x 10^8
Hope this helped, if you don't understand what I was doing in the working, you can just imagine substituting a = 10^8, things will make more sense. As for what I was talking about in the huge paragraph above, if you're confused you can also draw it out.
