Answer:
30
Step-by-step explanation:
So we begin by assigning variables to each side with:
DE = x
EF = y
DF = z
Now we start listing a system of equations from what is given:
x+y+z=81
x=2y
x-4=z
Once we solve that with substitution, we get the result of:
x=34
y=17
z=30.
Since z is the length of DF, the length of DF is 30.
I hope this helped! :D
