2x+2x=3x+5
4x=3x+5
x=5
CD = 5
Because C is the midpoint, CD would be half of BD. To find the full length of the segment BD you have to add the length of Cb and CD. Since CD is half you just double it. CB is the same length as CD. Because CB and CD together are the same as BD you make 2x+2x equal BD which is 3x+5. Once you have the equation you solve for x by adding 2x and 2x to get 4x. Then you subtract 3x from both sides to get x=5.