Well, we should first decide on our variables. Let's replace "thumb drives" with the letter "t", and "compact disk" with the letter "c".
Now, to understand the sentence. "5 thumb drives have 9 gigabytes more capacity than 1 compact disk." So, what does that mean? Well, if a compact disk has 10 gigabytes of capacity, then I suppose 5 thumb drives would have 19 gigabytes of capacity all together. So, we're just adding +9.
Alright, so as a linear equation:
"5 thumb drives have 9 gigabytes more capacity than 1 compact disk"
5t = c + 9