a. you would need to know how many toppings are going on the pizza. the price changes depending on the number of toppings, because each one adds 35 cents.
b. x equals the number of toppings. the number of toppings will change from pizza to pizza, so you put a variable in its place so you can use the same equation for every pizza.
c. 12 + 0.35x this is because the base price of every pizza is $12, then you add the number of toppings (x) multiplied by the price per topping ($0.35).
d. 12 + 0.35x when x=3 is 12 + 0.35(3) = 12 + 1.05 = $13.05