I believe the answer is 6sqrt2. You don’t really need to calculate, as you only need to know the rules about special triangles. From the image above, the triangle with side length of 4 inches is a 45 45 90 right triangle. You can prove this but I’m not going to go I depth unless you want me too. From this, you can also prove that the triangle with side length of 6 inches is also a 45 45 90 triangle. You can find the missing side length using the rule that in a 45 45 90 triangle, the hypotenuse is xsqrt2, which is basically 6sqrt2 in this case.