So for this, we will be using the pythagorean theorem, which is
![leg^2+leg^2=hypotenuse^2](https://tex.z-dn.net/?f=leg%5E2%2Bleg%5E2%3Dhypotenuse%5E2)
. 9 is being used instead of 18 because we are using only one half of this triangle.
Firstly, apply the numbers to their place
![9^2+20^2=x^2](https://tex.z-dn.net/?f=9%5E2%2B20%5E2%3Dx%5E2)
Solve the exponents to get
![81+400=x^2](https://tex.z-dn.net/?f=81%2B400%3Dx%5E2)
Add up the left half of the equation to get
![481=x^2](https://tex.z-dn.net/?f=481%3Dx%5E2)
And lastly, square root both sides of the equation, and your answer should be