please explain how you did it
[but if not thats fine too :)]

Answer:
D. 3
Step-by-step explanation:
The sides with lengths 25 and 5 are corresponding, so their ratio is 25/5 = 5/1
The sides with lengths 18 - x and x are also in a 5 to 1 ratio.
5/1 = (18 - x)/x
5x = 18 - x
6x = 18
x = 3
Answer: D. 3