What is the equation and the 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

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