Examples of this are:
2 and 
2 and 
4 and 
I'm guessing you have a work sheet going along with this which has actual images of containers. I've attached an example - if you can post the actual work sheet you're referring to I can edit this answer to correctly reflect your specific question.
Answer:
54
Step-by-step explanation:
Move the x's to the same side
2x + x = 8 +6
then simplify
3x = 14
make x stand on its own
(3x)/3 = 14/3
the 3's on the x side cancel
x = 14/3
mean = sum/n
mean = (5 + 7 + 7 + 9 + 12 + 16 + 21)/7
mean = 77/7
mean = 11