To solve this you must use Pythagorean theorem:
a and b are the legs (the sides that form a perpendicular/right angle)
c is the hypotenuse (the side opposite the right angle)
In this case...
a = 5
b = 8
c = x
^^^Plug these numbers into the theorem
simplify
25 + 64 =
89 =
To remove the square from x take the square root of both sides to get you...
√89 = x
(choice A)
Hope this helped!
~Just a girl in love with Shawn Mendes