So I'm using Khan Academy and I'm confused about what they did to get 72. What did they multiply or divide to get that?!?!

Solve the system of equations. y= 2x y= x^2 - 15

Bill works as a waiter and is keeping track of the tips he ears daily. about how much does bill have to earn in tips on sunday if he wants to average $22 a day? tips by day tips (dollars) monday tuesday wednesday thursday friday saturday $14 $22 $28 $36

2/3(-6y+9x) expand each expression using the distributive property