ABC is similar to DEF. Find x.

Through using the pythagorean theorem, you should find the hypotenuse of the smaller triangle to be 10 units. Therefore, the perimeter of the smaller triangle is 6+8+10 = 24 units.
The perimeter of the larger triangle is 60 units. The ratio here is 24:60 = 2:5
Meaning that the larger perimeter is 5/2 = 2.5 times that of the smaller.
We apply this scale factor 2.5 to the smaller hypotenuse 10 to jump to 2.5*10 = 25, which is the length of the larger hypotenuse.