For the present problem, it is already given a fixed value for the soda, and the money spent by Mallory buying pizza depends on the number of pizzas she bought. If X stands for the number of pizzas she bought, X x 10.95 is the price she paid for the pizzas. It allows us to write it as a function, where Y is the total of money she spent.
Because we already know Y is 89.22, we can substitute it, and we find: