Wow this will be late. Surprised no one answered this when it was on the regular list.
Pythagorean Theorem states a² + b² = c² where the sum of the squares of the two legs of a triangle (a and b) are equal to the square of the hypotenuse (c).
5² + b² = 13²
25 + b² = 169
b² = 144
b = 12
The length of the unknown leg labeled x is 12 units.
