If we call the leg of the small triangle y, and the leg perpendicular to x z, we can apply Pythagoras 3 times:
(1) 4 + y^2 = x^2
(2) x^2 + z^2 = 196 => z^2 = 196-x^2
(3) y^2 + 144 = z^2 => y^2 = z^2 - 144
Now fill in (3) in (1):
4 + z^2 - 144 = x^2
and then (2) in (1):
196-x^2-140 = x^2 => 2x^2 = 56 => x = √28 = 2√7
Answer C!
