Answer:
expansion
Step-by-step explanation:
I believe that the biggest reason for the general upward trend in the stock price is expansion. Amazon has expanded their company immensely over the last 5 years. They have also expanded into different markets such as drones, gaming, cloud streaming, video streaming, etc. All of these expansions and acquisitions have been incredibly profitable for Amazon and therefore reflect on the stock price. This also gives investors faith that the future of the company will continue growing which causes them to buy more stocks and therefore increase the price.
Hello!
If y varies jointly with x and z, it'll mean that x and z are being multiplied by a factor "k", to equal y.
To give you a visual, this is what I mean.

That means we'll want to find
with the first point we're given: 



Which means our equation is:

Now, let's plug in the
and
from the other point to find
.


Hope this helps!
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72 and 50 are the answer for 12 times 6 and 10 times 5