Let's recall the Pythagorean Theorem:
[tex]a^2+b^2=c^2\text{ }\Rightarrow\text{ c = }\sqrt{a^2+b^2}[/tex]
Where, a and b are the legs and c is the hypotenuse.
In our exercise, we know the value of the hypotenuse and one of the legs, then we calculate the length of the second leg, this way:
[tex]b\text{ = }\sqrt{13^2\text{ - }5^2\text{ }}\text{ }\Rightarrow\text{ b = }\sqrt{144}[/tex]
b = 12 units
