I'm having trouble solving this I don't know the steps what is 21x+6+90=180
I don't know if its that problem exactly but I need an answer and an explanation

Ms. aniston asked seven students in her class to collect pet food cans for a local animal shelter. the dot plot shows the number of food cans collected by each student. each dot represents one student. what is the median of the data set? a. 6 b. 9 c. 7 d. 8