Answer:
EF is 16.  
Step-by-step explanation:
So we know that Point E is somewhere on Line DF. In other words, DE plus EF must equal DF. In an equation, this is: 
[tex]DF=DE+EF[/tex]
We are given that DE is 2x, EF is 2x-6, and DF is 3x+5. So, substitute: 
[tex](3x+5)=(2x)+(2x-6)[/tex]
Now, solve for x. 
On the right, combine like terms: 
[tex]3x+5=4x-6[/tex]
Subtract 4x from both sides: 
[tex]-x+5=-6[/tex]
Subtract 5 from both sides: 
[tex]-x=-11[/tex]
Divide both sides by -1: 
[tex]x=11[/tex]
Now that we know x is 11, substitute this back into the equation for EF to find EF. 
[tex]EF=2x-6[/tex]
Substitute 11 for x: 
[tex]EF=2(11)-6[/tex]
Multiply: 
[tex]EF=22-6[/tex]
Subtract: 
[tex]EF=16[/tex]
And we're done!