Answer: 8 cm
Explanation:
The figure in the image attached below shows that there are two specific angles that are congruent to each other, angles AD and CD.
We are given the length of one of these angles:
AD= 4 cm so we must multiply 4 by 2, since there are TWO angles measuring 4 cm.
4 cm x 2 angles (AD and CD) =8 cm.
Proof of answer is shown below!