what is x? plz answer only if correct :)

Since this is a right triangle, the sides are related by the Pythagorean theorem. a^2+b^2=c^2

Legs are a and b. Hypotenuse is c. Plugging in the values gives you:

9^2 + 12^2 = x^2
81 + 144 = x^2

X^2 = 225

Taking the square root of both sides gives you x=15