Find the value of x.

x = √21.

Step-by-step explanation:

Triangles ADB and BCD are similar.

So their corresponding sides are in the same ratio:

x / 7 = 3 / x

x^2 = 21

x = √21.

D

Step-by-step explanation

The  problem says that Triangle ABC is dilated by a factor of two. This means the Triangle was cut in size down by half, so we can rule out any triangle smaller than that one. Which only leaves one answer since you've already chose A, so the correct answer is D.