To solve this you need to know Pythagorean theorem.
First, EG is 24, so the halfway points are 12. Knowing Pythagorean triples, you can use 5,12,13 and 12,16,20.
DF = 5+16
DF = 21
If you don't know Pythagorean triples, I have worked it out on the image attached.
First, EG is 24, so the halfway points are 12. Knowing Pythagorean triples, you can use 5,12,13 and 12,16,20.
DF = 5+16
DF = 21
If you don't know Pythagorean triples, I have worked it out on the image attached.