With the given information we know that the right angle is angle J, and the tangent of angle k is sqrt(5) / 4. Tangent ratio is the opposite side/adjacent side, so the opposite side of angle K is sqrt(5) and the adjacent side of angle K is 4.
To find the hypotenuse we use Pythagorean Theorem a^2 + b^2 = c^2.
(sqrt5)^2 + 4^2 = c^2
5 + 16 = c^2
21 = c^2
sqrt(21) = c
