<PRE><B>Question 324992</B>
I think you mean find x and y since you already know the lengths of the sides. 
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AB=CD
BC=DA
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1.{{{5x + 3y=31}}}
2.{{{3x + 2y=20}}}
Multiply eq. 1 by -2 and eq. 2 by 3 and add them,
{{{-10x-6y+9x+6y=-62+60}}}
{{{-x=-2}}}
{{{highlight(x=2)}}}
Then use either equation to solve for y,
{{{10+3y=31}}}
{{{3y=21}}}
{{{highlight(y=7)}}}</PRE>