So like how do I do this?

Answer:
Step-by-step explanation:
The legs of the triangle have lengths 9 and 12. The Pythagorean Theorem says [tex](\text{hypotenuse})^2=(\text{leg})^2+(\text{leg})^2[/tex].
[tex]x^2=9^2+12^2=81+144=225\\x=\sqrt{225}=15[/tex]
Yes, these values make a Pythagorean Triple (3 integers that fit the Pythagorean Theorem)
There are infinitely many of these triples: 3,4,5 5,12,13 are examples. The triple in this problem is 3 times the 3,4,5 triple.