The midpoint splits a segment in two sub-segments of equal length. This means that CD and DE are both half of CE
So, you have
[tex] CE = 2CD [/tex]
Plug in the expressions you're given:
[tex] 5x-1 = 2(x+7) \iff 5x-1 = 2x + 14 \iff 3x = 15 \iff x = 5 [/tex]
