Sarah wants to order a pizza with 2 different toppings. She has 9 different toppings to choose from. How many different pizzas could she order?
There are 9 choices of toppings, and we need to choose 2 distinct toppings. This is represented by the number of 2-element subsets of a 9-element set. We use the binomial coefficient ${9 \choose 2} = \boxed{36}$ to compute this.