Using the Pythagorean theorem,
.. x^2 = 3^2 +3^2 = (3^2)*2
.. x = √((3^2)*2)
.. x = 3√2 . . . . . . . . matches selection (a)
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The hypotenuse of an isosceles right triangle is always √2 times the length of either leg. Since the legs are 3, the hypotenuse is 3√2. You only need to remember this fact to save a bunch of arithmetic.