Question 81578
If the rectangle is a square, than all of the sides (x) are equal.  Since the triangles formed are right triangles and the hypotenuse is the diagonal, use the Pythagorean Theorem to solve: 
a^2+b^2=c^2
.

a=one side=x
b=another side=x
c=the hypotenuse=3
.
Plug-in the values and solve for the x-term
x^2+x^2=3^2
2x^2=9
2x^2/2=9/2
x^2=9/2
{{{(sqrt(x^2))}}}={{{(sqrt(9/2))}}}
x={{{(3/sqrt(2))}}}
x={{{(3(sqrt(2))/sqrt(2)sqrt(2))}}}  [rationalize the denominator]
x={{{(3(sqrt(2))/2))}}}

.
check by plugging all of the values back into:
a^2+b^2=c^2
{{{(3(sqrt(2))/2)^2)}}}+{{{(3(sqrt(2))/2)^2)}}}=3^2