Use the Pythagorean Theorem: (leg)^2 + (leg)^2 = (hypotenuse)^2, often written [tex]a^2+b^2=c^2[/tex] where a and b are the lengths of the legs of a right triangle, and c is the length of the hypotenuse.
In this problem, a = 9 and b = 12.
[tex]c^2=9^2+12^2=81+144=225 \\ c=\sqrt{225}=15[/tex]
