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I am thinking that the quadrilateral is not given to be a rhombus.
The perimeter is the sum of the 4 sides.
Adding 2x^2 + x -1 to 3x^2 -5x and then that sum to 4x^2 gives
9x^2 - 4x - 1 as the sum of the three given sides.
Subtract that from the given perimeter to get the missing side length.
5x^2 + 2x + 1 - (9x^2 -4x -1) =
-4x^2 + 6x + 2