Help me with this, it's Pythagorean theorem.

Step-by-step explanation:
Let x represent the missing side length.
By Pythagoras' Theorem,
We have x² + 12² = 13².
Therefore x² = 13² - 12² = 25 and x = 5.
(x = -5 is rejected as side lengths are positive)
Answer:
Let the required side of the triangle be x
By Pythagoras theorem we get,
[tex]x = \sqrt{ {13}^{2} - {12}^{2} } \\ = \sqrt{169 - 144} \\ = \sqrt{25} \\ = \sqrt{ {5}^{2} } \\ = 5[/tex]