(a)
First Restaurants
Let x be number of pizzas purchased
Total charge = 12x + 9
Second Restaurants
Let x be number of pizzas purchased
Total charge = 13x + 1
(b)
Total would be equal when
12x + 9 = 13x + 1
collect like terms
13x - 12x = 9 - 1
x = 8
Total charge for both restaurants will be equal when the total number of pizzas purchased is 8