How to find the average daily balance

Giuseppi's pizza had orders for $931.00 of pizzas. the prices were $21 for a large pizza, 514 for a medium pizza, and $7 for a small pizza. the number of large pizzas was two less than four times the number of medium pizzas. thenumber of small pizzas was three more than three times the number of medium pizzas. how many of each size of pizza were ordered?

If x is 6, then 7x= a. 13 b. 42 c. 42x d. 76

Which geometric principle is used to justify the construction below?