So the exercise I'm trying to solve goes like this: P ⊂ $R^2$ is the interior of a parallelogram defined by its' corners coordinates: (-2,1), (-1,3), (-3,3), (-4,1) I need to find out $vol_2 (P)$
Now I found somewhere in a book that if the set I'm looking at is
$P=[a_1,b_1] \times [a_2,b_2] \times ... \times [a_n,b_n]$ ⊂ $R^n$
then
$vol_n (P)$=$|b_1-a_1| |b_1-a_1|...|b_n-a_n|$
And I believe this finding may prove to be useful in what I need to solve, but I fail to identify the terms $[a_1,b_1]$ and $[a_2,b_2]$ in my scenario. Also, I don't really understand the concept of having a volume if this is a 2 dimensional figure.