Because it is a right triangle, you should use the Pythagorean theorem:
a^2 + b^2 = c^2
Due to the fact that you have the length of the hypotenuse and one leg, you need to slightly alter the equation:
a^2 = c^2 - b^2
Then, insert your variables:
a^2 = 13^2 - 4^2
Simplify:
a^2 = 169 -16
a^2 = 153
Finally, find the square root:
a = 12.4
Your missing leg is equivalent to 12.4
