Hello!
To find the side length you use the equation
[tex]a = \sqrt{2} \frac{d}{2} [/tex]
a is side length
d is diagonal
Put in the number we know
[tex]a = \sqrt{2} * \frac{27}{2} [/tex]
Divide
[tex]a = \sqrt{2} * 13.5[/tex]
Multiply
a = 19.091
The answer is 19.09
Hope this helps!