Answer:
If the hypotenuse of the 30 60 90 triangle is 7 sqrt 3, then the length of the longer side is equal to 7 x 3, or 21.
Now going to the 45 45 90 triangle, we can see that x is equal to 21[tex]\sqrt{2}[/tex]. This is because the hypotenuse of a 45 45 90 triangle is equal to the side length times sqrt 2.
So our answer is, 21[tex]\sqrt{2}[/tex].
Let me know if this helps!