In a right triangle with a 45° angle, the lengths of the two legs are equal, and if we let \( x \) represent the length of each leg, then the hypotenuse \( h \) can be calculated using the Pythagorean theorem:
\[ h = x\sqrt{2} \]
For the second triangle, we know the length of the side opposite the 45° angle is \( \sqrt{42} \). Therefore, both legs of this triangle are \( \sqrt{42} \), and we can find the hypotenuse as follows:
\[ h = \sqrt{42}\sqrt{2} = \sqrt{42 \cdot 2} = \sqrt{84} \]
Next, we can simplify \( \sqrt{84} \):
\[ \sqrt{84} = \sqrt{4 \cdot 21} = 2\sqrt{21} \]
Thus, the length of the hypotenuse in the second triangle is \( 2\sqrt{21} \).
However, it seems that none of the provided responses match this result. The closest we can identify from the available options based on the construction of the triangles under standard conditions is that the hypotenuse of the second triangle does not correspond to any of the responses. Please double-check if there is a specific context or condition you may want to mention additionally for clarification.