For the given parallelogram to be a square, its diagonals should be equal. Let's put more details in the given figure to better understand:
Naming the two diagonals to be AC and BD, for it to be a square,
AC = 12x - 23
BD = 4x + 17
We get,
For the diagonals to be equal, x = 5.
Therefore, for it (parallelogram) to be a square, x should be equals to 5.
Let's check,
AC = 12x - 23 = 12(5) - 23 = 60 - 23 = 37
BD = 4x + 17 = 4(5) + 17 = 20 + 17 = 37