Question 290097
The diagonal creates a right triangle with two sides being the sides of the square. Let x be the length of the side:

Then by the Pythagorean Theorem we have:

x^2 + x^2 = 12^2
2x^2 = 144
x^2 = 72
x = sqrt(72)
x = sqrt(36*2)
x = sqrt(36)*sqrt(2) 
x = 6*sqrt(2)