I would really appreciate it if you can help me with with this problem and explain how u got the answer please and thank you here is the problem:

Which property was used to simplify the expression? 3c+9 +4c=3c+4c+9

Sophia wrote an equation to represent the revenue of a movie theater for one day. she let x represent the number of child tickets sold and y represents the nunver of adult tickets sold. if a child ticket cost $4, an adult ticket cost $6, and the total revenue for the daybwas $420, which equation could sophia use to represent the number of chikd and adult tickets that were sold?

What is the domain of the step function f(x)=[2x]-1?