In a right traiangles, note that the longest side or the side facing the 90degrees is the hypotenuse.
Also, the side facing a given angle is the opposite
Then, the last side is adjacent.
Also, note the pythagorean theorem that;
Also note the concept below;
As explained y represents the hypotenuse because it is the longest side,
x represents the opposite side because it faces the given angle,
so 13 is the adjacent.
First, using TOA concept to get x;
On getting x=13, the pythagoream theorem can be used to get the third side called the hypotenuse.