[1,2,3]
[1,3,6]
[3,8,6]
[3,7,3]
I know I have to find 2 (I think) vectors that determine the area and then I take the cross product of the 2. That's about all I know I don't know which 2 determine the area.
The given vectors are the stationary vectors of the vertices of the parallelogram. Calculate the vectors describing the sides of the parallelogram:
$\displaystyle \overrightarrow{AD} = \vec d - \vec a = [2,5,0]$
$\displaystyle \overrightarrow{BC} = \vec c - \vec b = [2,5,0]$
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$\displaystyle \overrightarrow{AB} = \vec b - \vec a = [0,1,3]$
$\displaystyle \overrightarrow{DC} = \vec c - \vec d = [0,1,3]$
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You now know the pairs of parallels. As you have written, the area is
$\displaystyle a_{parallelogram} = |\overrightarrow{AD} \times \overrightarrow{AB}| =| [2,5,0] \times[0,1,3] |= | [15,-6,2] | = \sqrt{265} \approx 16.2788$