We can find what we need with the following system of equations:
x + y = 210,
12x + 15y = 2910,
where x is the number of small pizzas that were sold and y is the number of large pizzas that were sold.
Let's say x = 210 - y and substitute it into the second equation to get
12(210 - y) + 15y = 2910.
We can use this to solve for y and get y = 130 (large pizzas). This means x = 210 - 130 = 80 (small pizzas).