If we look at his calculations
2((9)(20) is the front an back rectangle
2((9)(34)) is the side rectangles
(34)(20) is the bottom
2(1/2(20(24)) is te front and top triangles
and the last one assumes that they are triangles which is incorrect
answer is D
Given that D is the mid point of CE ,
So D will bisect CE, it means the length of DE and CD is equal.
=> DE + CD = CE
As DE and CD are equal,
=> DE + DE = CE
=> 2 DE = CE
Putting values from the question,
=> 2( 7x - 1 ) = 10x + 18
=> 14x - 2 = 10x + 18
=> 14x - 10x = 18 + 2
=> 4x = 20
=> x = 20 / 4
=> x = 10 / 2
=> x = 5

Therefore, the value of x is 5