Are you familiar with functions?
s(p) is a notation. The letter of the function is s. When it says s(10), that means: "plug in the 10 in every variable p".
They said that s is the number of cartridges sold (in thousands) and p is the price per cartridge. They also said that at the price $10, 100000 cartridges were sold.
To solve this question, you must find the constant a using the given value of s(10), then find s(20).