You have the right idea that things need to get multiplied.
What should be done is that the entire fraction needs to get multipled by the lowest common denominator of both denominators.
Let's look at the complex numerator. Its denominators are 5 and x + 6. Nothing is common with these, so both pieces are needed.
The complex denominator has x - 3 as its denominator. With nothing in common between it and the complex numerator, that piece is needed.
So we multiply the entire complex fraction by (5)(x + 6)(x -3).
Numerator: 
= (x+6)(x-3) - (5)(5)(x-3)
= (x+6)(x-3) - 25(x-3)
= (x-3)(x + 6 - 25) <--- by group factoring the common x - 3
= (x -3)(x - 19)
Denominator:

Now we put the pieces together.
Our fraction simplies to (x - 3) (x - 19) / 125 (x + 6)
Step-by-step explanation:
All you need to do is divide cost by mile since they already factored in the additional $3 for you.
1. 8/5 = $1.60
2. 11/8 = $1.375 = $1.38
3. 23/20 = $1.15
x + 6 = x
there is literally no solution.
Easy, sub -3 for x
f(-3)=3(-3)-10
f(-3)=-9-10
f(-3)=-19
the value of f(-3) is -19