Question 944558
Not being a full solution answer, but follow this way:


Your unknown point D, will have slope for DC equal to slope for AB.  Also, slope for BC will be same as slope for AD.  Now you would know the slopes for all four segments of the parallelogram.


D is the intersection of two lines.
Slope for BC containing point C;
and
slope for AB containing point A.
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Use point-slope equational forms to make those two equations, and find their point of intersection.   That will be point D.