As all sides of a square are equal, and the perimeter is simply the sum of the sides, all you need to do is divide 68 by four, which gives you 17.
Now we know that 8x - 3 = 17, we just need to rearrange it to get x.
We can start by adding 3 to both sides to get 8x = 20.
Finally, divide 20 by 8 to get x. 20 divided by 8 is 2.5.
x = 2.5