Question 171781
Since the diagonals are equal, this means RT = QC 
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3x^2 = 5x+4
3x^2 - 5x - 4 = 0
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Since you can't factor you must resort to the quadratic equation.  Doing so will yield the following solutions:
x =  {2.257, -0.591}
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You can toss out the negative solution leaving:
x = 2.257
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Details of quadratic follows:
*[invoke quadratic "x", 3, -5, -4 ]
(3x+2)(x-2) = 0