Since this is a 45-45-90 right triangle, we can use one of it's properties to easily find the hypotenuse.
In a right triangle like this, the hypotenuse always has a value that's √2 times larger than the legs (the other sides).
Thus, let's multiply 3√2 by √2.
(3√2)(√2)
= 3√4 = 3*2
= 6
That should be your answer. Happy studying~