To find the diagonal of the square,we would first need to find it's length,which can be find by:
12÷4=3ft
To find diagonal,we can use the Pythagorean Theorem:
[tex] \sqrt{ {3}^{2} +  {3}^{2}  }   \\  =  \sqrt{18}   [/tex]
Which can also be written as ≈4.243
Thus,the answer could be: _/18 or 4.243
Hope it helps!