In a square, all sides are equal to each other. So, therefore, you can set both equations given equal to each other. Doing that you will get
6x-1=4x+6
From here you can begin to solve the equation (Move the whole numbers to one side and the x’s to the other).
When you do that you get
2x=7
Solve for x and you get
X=7/2 or 3.5
Now that you have the value of x you can plug it into the equation and solve for the length of the side
6x-1
6(3.5)-1=20cm
For both equations you should get the same result
4x+6
4(3.5)+6=20 cm
So the length is equal to 20cm
Hope this helps