Answer is 21.4 m for unknown side length
Using Pythagorean Theorem we can find the missing side, the hypotenuse.
Let c = the missing side
Let your equation be a^2 + b^2 = c^2
13^2 + 17^2 = c^2
169 + 289 = c^2
458 = c^2
√458 = √c^2
21.4 = c
Your answer is 21.4 m.