A square has 4 equal sides. If the perimeter of this square is 6x-20 units, then the side length is 1/4 of that:
6x-20 6x 20
side length = ----------- = ---- - ----- = 3x/2 - 5
4 4 4
Unfortunately, there's not enough info given here to calculate x. Again, please share the illustration of this square. Probably the needed info is there.
