Question 1034666
When you cut the property in half with a diagonal line you end up with two identical triangles.
Because we are dividing a square, the resulting triangles with have angles of  45, 45, and 90 degrees.
These triangles, called 45-45-90 triangles, have special properties. The hypotenuse is equal to the sqrt 2. So the length of the cable is √2 = 1.414.
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NOTE: OF course, you can use the Pythagorean theorem to find it: 
a^2+b^2 = c^2
But why would you, when there is a standard mathematical answer.