It is obvious that Triangle BCD is congruent to Triangle EFG. EFG is twice the size of BCD.
Put the lengths into the pythagorean theorem equation. (the longest side is the hypotenuse)
So for Triangle BCD
3^2+b^2=5^2
(3×3)+b^2=(5×5)
9+b^2=25
So what plus 9 equals 25?
16
the square root of 16 is 4 so 4^2=16
9+16=25
Now since we know that Triangle EFG is twice the size of BCD remake the equation but the numbers will be:
6^2+8^2=10^2
Now check
(6×6)+(8×8)=(10×10)
36+64=100
100=100
So for Triangle EFG X=8