Answer:
D. 13
Explanation:
From the diagram, 
 and
 and 

In an isosceles trapezium, the base angles are equal.
This implies that 
 
 

The side length CB of the trapezoid is a transversal line because CD is parallel to AB.
This means that 
 and
 and 
 are co-interior angles.
 are co-interior angles.
Since co-interior angles are supplementary, we write and solve the following equation for 
 .
.
 
 
 
Group similar terms
 
 
 
Simplify both sides of the equation.
 

Divide both sides by 12 
 


The correct answer is D.