Answer:
no the quadrilateral isn't a parallelogram
Step-by-step explanation:
For it to be a parallelogram, the opposite sides must be parallel to each other and in this case it's not.
A right triangle :) to find the length of a missing side, you can use the pythagorean theorem (a^2 + b^2 = c^2)
Multiply 12 x 12 which is 144. And then you divide that by 6 and you get 24

Hope you could get an idea from here.
Doubt clarification - use comment section.
(y_final-y_start)/(x_final-x_start) always! :) that easy
(-6-2)/(2-0) = -4
You probably know that if the interval was pretty small, between 0 and 0.0001, it would take you to the concept of slope and derivatives, but average change is just ignoring anything in between x=0, and x=2 (it could go up/down million times, and still the answer would be -4)
hope it helps