If E is the midpoint Of DF, that means DE is the same length as EF. Therefore, you can make DE=EF or 6x+8=7x-5 and solve from there.
6x+8=7x-5
+5 +5
6x+13=7x
-6x -6x
13=x
Now substitute 13 for x in 6x+8 to find DE.
6(13)+8 = 86
We already know since both sides are equal that EF also equals 86, but you can still substitute 13 again in 7x-5 to get EF.
7(13)-5 = 86
Now that we have both halves of DF, we can add 86+86 to get our total length of 172.
