Answer:
x=3
Explanation:
Use the Pythagorean Theorem to write an equation.
x^2+y^2=z^2
Substitute values from the problem.
x^2 + 6^2 = 9^2
Solve for what you know.
x^2 + 36 = 81
Square root it.
x+6=9
Subtract 6 from both sides.
x=3
In the future, if you see a right triangle with an unknown side, and the other two sides are either 3, 6, or 9, you know that the other one is the missing value out of 3/6/9. This is called a 3/6/9 triangle.