You will have to graph it to see that AB and CD are opposite sides, and AD and BC are opposite sides. The coordinates are (x,y) and the slope of each side is (y2-y1)/(x2-x1).
AB and CD must have the same slopes. For AB it's (9-5)/(5-2) = 4/3 and for CD it's (-1-3)/(3-6) = -4/-3 = 4/3. Those sides are parallel.
AD and BC must have the same slopes. For AD it's (5-(-1))/(2-3) = 6/-1 = -6 and for BC it's (9-3)/(5-6) = -6. That figure is a parallelogram because the opposite sides are parallel.
Graph 2. It's not a parallelogram.
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