Answer
CD = 5 units
Explanation
It is given that
BC = 12x
CD = x + 4
BD = 17
And we are told to solve for CD.
From the diagram, we can see that
BD = BC + CD
Or better written as
BC + CD = BD
Substituting BC, CD and BD with the expressions given, we can easily solve for x
12x + (x + 4) = 17
12x + x + 4 = 17
13x + 4 = 17
Subtract 4 from both sides
13x + 4 - 4 = 17 - 4
13x = 13
Divide both sides by 13
(13x/13) = (13/13)
x = 1
Now that we know x, we can easily calculate what CD is
CD = x + 4 = 1 + 4 = 5 units
Hope this Helps!!!