If D is the midpoint of CE, that means that D is exactly in the middle of CE, splitting it in 2 equal length segments. That means that CD = DE in length. If CD = 9x - 7 and DE = 3x + 17, then
9x - 7 = 3x + 17 and
6x = 24 so
x = 4
Now we'll sub 4 in for x in either expression for half the length. If DE = 3x + 17, then when we sub in 4 for x: 3(4) + 17 = 29. That means that half the length of CE is 29. Double that and you'll get that the length of CE = 58
