Question 74673
(1) If the given points are 
({{{x[1]}}},{{{y[1]}}})
({{{x[2]}}},{{{y[2]}}}) (the middle point)
({{{x[3]}}},{{{y[3]}}})
Find the slopes of 2 adjacent sides using
{{{m[1] = (y[2] - y[1]) / (x[2] - x[1])}}}
{{{m[2] = (y[3] - y[2]) / (x[3] - x[2])}}}
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(2) Find ({{{x[4]}}},{{{y[4]}}}) using
{{{m[1] = (y[4] - y[3]) / (x[4] - x[3])}}}
{{{m[2] = (y[4] - y[1]) / (x[4] - x[1])}}}
(These are 2 equations with 2 unknowns)
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If you draw the parallelogram and label points and
label the parallel sides m1 and m2, this should make sense.