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

Sera sells t-shirts at the beach. she believes the price of a t-shirt and the number of t-shirts sold are related. she has been experimenting with different prices for the t-shirts. she has collected a data set with five pairs of data; each consists of the price of a t-shirt and the number of shirts sold. the independent variable, which will go on the x-axis, is . the dependent variable, which will go on the y-axis, is the