Answer:
A. 21
Explanation:
This is a 30-60-90 triangle, where you have one 30° angle, one 60°, and one 90° (aka right) angle. 
The sides of a 30-60-90 triangle adhere to the following rules:
- The side opposite the 30° angle is the shortest side and we can call its length "x".
- The side opposite the 60° angle is the medium length side and its length is given by x * √3. This means its length is the product of the length of the side opposite the 30° angle and √3.
- The side opposite the 90° (right) angle is the longest side (aka the hypotenuse) and its length is given by 2x. This means its length is twice the length of the side opposite the 30° angle.
Since 7√3 is the length of the side opposite the 30° angle, the entire expression represents x.
Since the length of the side opposite the 60° angle is given by x * √3, the length of this side is (7√3)(√3).
Simplifying gives us 7*3, which is 21.
Thus, the value of x in the triangle is 21 (answer choice A.)