Hello!
You can use the Pythagorean Theorem to solve this
[tex]a^{2} + b^{2} = c^{2} [/tex]
c is the hypotenuse which is the side across the right angle
a and b are the other sides
Put in the values you know
[tex] 9^{2} + 12^{2} = c^{2} [/tex]
Square the numbers
[tex]81 + 144 = c^{2} [/tex]
Add
[tex]225 = c^{2} [/tex]
Take the square root of both sides
15 = c
The answer is 15
Hope this helps!