This is a special right triangle with 3 sides that are 5, 12, 13. You should remember this triangle.
[tex] {13}^{2} = {x}^{2} + {12}^{2} \\ {x}^{2} = {13}^{2} - {12}^{2} \\ x = \sqrt{ {13}^{2} - {12}^{2} } = \sqrt{169 - 144} = \sqrt{25} = 5[/tex]
