Triangle area ABD is equal to: ̅ ̅̅ ̅ ̅ ̅̅ ̅̅
Triangle area ACD is equal to: ̅ ̅̅ ̅ ̅ ̅̅̅ ̅
Since two triangle areas is equal, than with twice one of them we find area of parallelogram.
̅ ̅̅ ̅ ̅ ̅̅̅ ̅
̅ ̅̅ ̅ ̅ ̅̅ ̅
The base of a high = area of parallelogram