distance(Mercury, Sun) = 5.8 x 10^7
distance(Venus, Sun) = 1.1 x 10^8
distance(Venus, Mars) = 1.1 x 10^8 - 5.8 x 10^7
The subtraction is a little easier if you borrow a power of 10 from the 10^8, multiplying 1.1 by 10.

simply subtract 5.8 from 11.0.
This gives us a result of 5.2 x 10^7.
Hope that makes sense!