This triangle is a Gandalf triangle: the 45-45-90 one. We can either use the Pythagorean theorem or the formula we derived from it. We should know where the formula comes from, so we'll re-derive it real quick.
a2 + b2 = c2
Since a = b in our case, we'll replace both with x.
x2 + x2 = 2a2 = c2
Just square root both sides, and presto.
Awesomesauce. Looking at our cut-in-half box, we know the length of one side. That would be x, and the hypotenuse is d.