Answer
B
Explanation
To find the value of x, you need to use the Pythagorean theorem. 

x is one of the legs. 
y is another one of the legs. 
z is the hypotenuse (the longest side of a right triangle).
Now for this we can set 5 to be x and 4 to be y. 
y is 4 because 4 is half of 8. We need to find half of 8 because the right triangle is half of the whole triangle. 
Now 'plug' these into the formula and find z. 

To find z then we must find the square root of 41. 
So the answer is B. 
(Now I just realized that I used the variable z for the hypotenuse when you were supposed to be finding x for the hypotenuse but whatever).