Answer:
D. 10.2
Explanation:
Since the triangle is a right triangle we can use the Pythagorean theorem. 
I will call the hypotenuse 
 , so in this case:
, so in this case: 

and one leg will be 
 , in this case:
, in this case: 

and the unknown for this problem is the other leg which in the image is called 
 .
. 
So the formula for the Pythagorean theorem:
 

clearing for x: 

substituting 
 and
 and 
 :
: 

the value of x is option D. 10.2