In the right angled triangle, a hypotenuse is the longest line of the triangle and is always the one that is opposite the right-angle of the triangle.
If your triangle is a right angled triangle, the simplest way is to use the Pythagora's Theorem to find the length of the hypotenuse. The formula for which is
a² + b² = c² where c is the unknown hypotenuse and a and b are the other two sides.
Using this formula,
9² + 12² = c²
89 + 144 = c²
233 = c²
√233 = √c²
15.25 = c which is you see is also the lengthiest..