Find the value of x for which abcd must be a parallelogram
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If it doesn't matter whether the points are integers or not you can just pick a number for either y or x and solve for the other variable. For example, if you pick 3 to be your x, plug it in and then solve for y. Start out with: 
Plug in 3 for x: 
. Then, 
. Subtract 21 from both sides and then divide by -3 on both sides. You end up with y=-7 so one of your points is (3, -7)